What size pot would a *mature clump* of Hesperaloe parviflora need to be healthy?

A neighbor who works as a landscaper grew many more from seed than he's been able to sell to clients, and he sold several to me cheap. I don't want the gophers to get them (they go after my Aloes & some other plants every year), but I do want them to grow into large clumps. So, I am wondering how to go about getting them to grow into large clumps IN pots. (Thus, my question about the pot's size.)