There's no basis on height or anything. The rollers generally come in standard lengths - 3ft, 1.5ft, etc. A bigger roller will work a large surface area and a smaller one a smaller area.
I started with a 3ft roller and it works well for most everything I do, but I've also used an 18" roller at the gym and that has worked well, too.
The big difference is in the density of the foam. A softer roller will give you a less intense foam rolling experience that a more dense one. When I first started foam rolling, the soft roller was plenty intense, but I have since graduated up to denser rollers, as well as tennis and lacrosse balls for rolling out some areas.
Hope this helps!