When you create an entity, you can specify it’s dimensions using the component attributes:
<a-box
width="2"
height="2"
depth="2"
></a-box>
This can also be represented using a single attribute for geometry:
<a-box
geometry="width: 2; height: 2; depth: 2"
></a-box>
When working with the <a-sphere> entity, you specify the size using the radius:
<a-sphere
radius="2"
></a-sphere>
If you want to create a non-uniformly sized sphere, you can use scaling to squash or stretch on any of the x/y/z axis.