Ours is about 4 x 6 and works great.

Ours is 4x7 (4x6 if you don't count the bench) and it works great too. Our shower heads are side by side. The benefit for 5x5 is that you can turn a wheelchair 360 degrees if needed in that space. Are you planning to stay in this house "forever"? Is one of you currently handicapped or have a degenerative disease that will make getting around difficult? If so, 5x5 may be a good idea. We figured with 4x6/4x7 you can wheel in forward & wheel out backward.

What does your space look like?