The easiest way to create a D3.js layout is to start with an <svg> element that acts as the container (i.e. backdrop) for your chart. Simply add something like:
<svg width="300px" height="300px"></svg>
To whatever width or height you choose. Then you can add other elements within it, for example:
<svg width="300px" height="300px">
<rect x="20" y="20" width="20px" height="20" rx="5" ry="5" />
<rect x="60" y="20" width="20px" height="20" rx="5" ry="5" />
<rect x="100" y="20" width="20px" height="20" rx="5" ry="5"/>
</svg>