There is no way to keep yuccas from growing to their biological height. There is a larger number of yuccas that stay "small" and "rounded." Google yucca, you find several/many site to view.

Go to websites for Plant Delights Nursery (NC), Yuccado (TX), and High Country Gardens (NM). They provide a lot of info on the eventual size of the yuccas they offer. Mary Irish's book on Agaves and Yuccas is also very good: you can get a used one on Amazon for cheap.

In case you don't get your hands on that book (which is great by the way), I'll just say that it seems the MAJORITY of Yucca species are short. They are usually separated into two groups: trunked (in an obvious way) and trunkless (or nearly so), the latter having the largest number of species, at least outside of the tropics. Here in NM, plains yucca (Y.glauca) is a very tough trunkless species that forms a nice rounded shape.