This is a good question!

I'm a tall girl. 5'11" and 140 lbs. I am pretty skinny, but have always thought of myself as big. After all, I'm taller than the majority of men and almost all women. I had no experience with feeling small, delicate, petite...until I met Him. Knowing for an absolute fact that He is bigger and stronger than me is a thrill.

It isn't a requirement, however. Just a nice added bonus in the power-exchange dynamic.