Ficus can be maintained in any pot size you wish. If the roots are filling the current container, and you don't want a larger container (and i wouldn't, too hard to manage in my opinion), then at the next repotting (this should be in the early summer, when it's warm and the plant is growing well), root prune about 1/3 of the roots, work out the old soil from the root ball, and repot with fresh soil into the same container. Water well, then not again until the soil is very dry or new growth resumes, at which point you can resume your normal care. Every time it gets root bound you repeat the process, but go ahead and let it get root bound, ficus don't mind until it gets ~really~ extreme. I would top prune about 1/3 of the foliage at the same time to control post transplant leaf drop, but some people would disagree. That's what works for me though.

Does anyone know if you can plant dichondra (silver falls) in with a rubber tree? I understood that dichondra doesn't require much water either...but will they be compatible?

Do you put your ficus outside in the summer? Im just curious whether its a good idea or not.