Ширина и высота холста в HTML5

var c = document.getElementsByTagName('canvas')[0];
var ctx = c.getContext('2d');
ctx.lineWidth   = 1;
ctx.strokeStyle = '#f00';
ctx.fillStyle   = '#eff';

ctx.fillRect(  10.5, 10.5, 20, 20 );
ctx.strokeRect( 10.5, 10.5, 20, 20 );
ctx.fillRect(   40, 10.5, 20, 20 );
ctx.strokeRect( 40, 10.5, 20, 20 );
ctx.fillRect(   70, 10, 20, 20 );
ctx.strokeRect( 70, 10, 20, 20 );

ctx.strokeStyle = '#fff';
ctx.strokeRect( 10.5, 10.5, 20, 20 );
ctx.strokeRect( 40, 10.5, 20, 20 );
ctx.strokeRect( 70, 10, 20, 20 );

body { background:#eee; margin:1em; text-align:center }
canvas { background:#fff; border:1px solid #ccc; width:400px; height:160px }

<canvas width="100" height="40"></canvas>
<p>Showing that re-drawing the same antialiased lines does not obliterate old antialiased lines.</p>

const ctx = document.querySelector("#c").getContext("2d");

function render(time) {
  time *= 0.001;
  resizeCanvasToDisplaySize(ctx.canvas);
 
  ctx.fillStyle = "#DDE";
  ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  ctx.save();
 
  const spacing = 64;
  const size = 48;
  const across = ctx.canvas.width / spacing + 1;
  const down = ctx.canvas.height / spacing + 1;
  const s = Math.sin(time);
  const c = Math.cos(time);
  for (let y = 0; y < down; ++y) {
    for (let x = 0; x < across; ++x) {
      ctx.setTransform(c, -s, s, c, x * spacing, y * spacing);
      ctx.strokeRect(-size / 2, -size / 2, size, size);
    }
  }
  
  ctx.restore();
  
  requestAnimationFrame(render);
}
requestAnimationFrame(render);

function resizeCanvasToDisplaySize(canvas) {
   // look up the size the canvas is being displayed
   const width = canvas.clientWidth;
   const height = canvas.clientHeight;

   // If it's resolution does not match change it
   if (canvas.width !== width || canvas.height !== height) {
     canvas.width = width;
     canvas.height = height;
     return true;
   }

   return false;
}

body { margin: 0; }
canvas { display: block; width: 100vw; height: 100vh; }

<canvas id="c"></canvas>

const ctx = document.querySelector("#c").getContext("2d");

function render(time) {
  time *= 0.001;
  resizeCanvasToDisplaySize(ctx.canvas);
 
  ctx.fillStyle = "#DDE";
  ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  ctx.save();
 
  const spacing = 64;
  const size = 48;
  const across = ctx.canvas.width  / spacing + 1;
  const down   = ctx.canvas.height / spacing + 1;
  const s = Math.sin(time);
  const c = Math.cos(time);
  for (let y = 0; y <= down; ++y) {
    for (let x = 0; x <= across; ++x) {
      ctx.setTransform(c, -s, s, c, x * spacing, y * spacing);
      ctx.strokeRect(-size / 2, -size / 2, size, size);
    }
  }
  
  ctx.restore();
  
  requestAnimationFrame(render);
}
requestAnimationFrame(render);

function resizeCanvasToDisplaySize(canvas) {
   // look up the size the canvas is being displayed
   const width = canvas.clientWidth;
   const height = canvas.clientHeight;

   // If it's resolution does not match change it
   if (canvas.width !== width || canvas.height !== height) {
     canvas.width = width;
     canvas.height = height;
     return true;
   }

   return false;
}

span { 
   width: 250px; 
   height: 100px; 
   float: left; 
   padding: 1em 1em 1em 0;
   display: inline-block;
}
canvas {
   width: 100%;
   height: 100%;
}

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Praesent cursus venenatis metus. Mauris ac nibh at odio scelerisque scelerisque. Donec ut enim <span class="diagram"><canvas id="c"></canvas></span>
vel urna gravida imperdiet id ac odio. Aenean congue hendrerit eros id facilisis. In vitae leo ullamcorper, aliquet leo a, vehicula magna. Proin sollicitudin vestibulum aliquet. Sed et varius justo.
<br/><br/>
Quisque tempor metus in porttitor placerat. Nulla vehicula sem nec ipsum commodo, at tincidunt orci porttitor. Duis porttitor egestas dui eu viverra. Sed et ipsum eget odio pharetra semper. Integer tempor orci quam, eget aliquet velit consectetur sit amet. Maecenas maximus placerat arcu in varius. Morbi semper, quam a ullamcorper interdum, augue nisl sagittis urna, sed pharetra lectus ex nec elit. Nullam viverra lacinia tellus, bibendum maximus nisl dictum id. Phasellus mauris quam, rutrum ut congue non, hendrerit sollicitudin urna.
</p>

const ctx = document.querySelector("#c").getContext("2d");

function render(time) {
  time *= 0.001;
  resizeCanvasToDisplaySize(ctx.canvas);

  ctx.fillStyle = "#DDE";
  ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  ctx.save();
 
  const spacing = 64;
  const size = 48;
  const across = ctx.canvas.width / spacing + 1;
  const down = ctx.canvas.height / spacing + 1;
  const s = Math.sin(time);
  const c = Math.cos(time);
  for (let y = 0; y < down; ++y) {
    for (let x = 0; x < across; ++x) {
      ctx.setTransform(c, -s, s, c, x * spacing, y * spacing);
      ctx.strokeRect(-size / 2, -size / 2, size, size);
    }
  }
  
  ctx.restore();
  
  requestAnimationFrame(render);
}
requestAnimationFrame(render);

function resizeCanvasToDisplaySize(canvas) {
   // look up the size the canvas is being displayed
   const width = canvas.clientWidth;
   const height = canvas.clientHeight;

   // If it's resolution does not match change it
   if (canvas.width !== width || canvas.height !== height) {
     canvas.width = width;
     canvas.height = height;
     return true;
   }

   return false;
}

// ----- the code above related to the canvas does not change ----
// ---- the code below is related to the slider ----
const $ = document.querySelector.bind(document);
const left = $(".left");
const slider = $(".slider");
let dragging;
let lastX;
let startWidth;

slider.addEventListener('mousedown', e => {
 lastX = e.pageX;
 dragging = true;
});

window.addEventListener('mouseup', e => {
 dragging = false;
});

window.addEventListener('mousemove', e => {
  if (dragging) {
    const deltaX = e.pageX - lastX;
    left.style.width = left.clientWidth + deltaX + "px";
    lastX = e.pageX;
  }
});

body { 
  margin: 0;
}
.frame {
  display: flex;
  align-items: space-between;
  height: 100vh;
}
.left {
  width: 70%;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
}  
canvas {
  width: 100%;
  height: 100%;
}
pre {
  padding: 1em;
}
.slider {
  width: 10px;
  background: #000;
}
.right {
  flex 1 1 auto;
}

<div class="frame">
  <div class="left">
     <canvas id="c"></canvas>
  </div>
  <div class="slider">
  
  </div>
  <div class="right">
     <pre>
* controls
* go 
* here

&lt;- drag this
     </pre>
  </div>
</div>

const ctx = document.querySelector("#c").getContext("2d");

function render(time) {
  time *= 0.001;
  resizeCanvasToDisplaySize(ctx.canvas);
 
  ctx.fillStyle = "#DDE";
  ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  ctx.save();
 
  const spacing = 64;
  const size = 48;
  const across = ctx.canvas.width / spacing + 1;
  const down = ctx.canvas.height / spacing + 1;
  const s = Math.sin(time);
  const c = Math.cos(time);
  for (let y = 0; y < down; ++y) {
    for (let x = 0; x < across; ++x) {
      ctx.setTransform(c, -s, s, c, x * spacing, y * spacing);
      ctx.strokeRect(-size / 2, -size / 2, size, size);
    }
  }
  
  ctx.restore();
  
  requestAnimationFrame(render);
}
requestAnimationFrame(render);

function resizeCanvasToDisplaySize(canvas) {
   // look up the size the canvas is being displayed
   const width = canvas.clientWidth;
   const height = canvas.clientHeight;

   // If it's resolution does not match change it
   if (canvas.width !== width || canvas.height !== height) {
     canvas.width = width;
     canvas.height = height;
     return true;
   }

   return false;
}

body { margin: 0; }
canvas { 
  display: block; 
  width: 100vw; 
  height: 100vh;  
  position: fixed;
}
#content {
  position: absolute;
  margin: 0 1em;
  font-size: xx-large;
  font-family: sans-serif;
  font-weight: bold;
  text-shadow: 2px  2px 0 #FFF, 
              -2px -2px 0 #FFF,
              -2px  2px 0 #FFF,
               2px -2px 0 #FFF;
}

<canvas id="c"></canvas>
<div id="content">
<p>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Praesent cursus venenatis metus. Mauris ac nibh at odio scelerisque scelerisque. Donec ut enim vel urna gravida imperdiet id ac odio. Aenean congue hendrerit eros id facilisis. In vitae leo ullamcorper, aliquet leo a, vehicula magna. Proin sollicitudin vestibulum aliquet. Sed et varius justo.
</p>
<p>
Quisque tempor metus in porttitor placerat. Nulla vehicula sem nec ipsum commodo, at tincidunt orci porttitor. Duis porttitor egestas dui eu viverra. Sed et ipsum eget odio pharetra semper. Integer tempor orci quam, eget aliquet velit consectetur sit amet. Maecenas maximus placerat arcu in varius. Morbi semper, quam a ullamcorper interdum, augue nisl sagittis urna, sed pharetra lectus ex nec elit. Nullam viverra lacinia tellus, bibendum maximus nisl dictum id. Phasellus mauris quam, rutrum ut congue non, hendrerit sollicitudin urna.
</p>
</div>