I'm an average size woman and I think it has been an advantage, especially at the beginning. When I first started, I would train a lot after class with my tall, strong guy friend who had started at the same time. It became VERY clear when my technique was wrong. When he would do a technique, however, sometimes it would work because he would muscle it. At higher levels, I wouldn't think it would make a difference. I doubt being 6'3 and 215 would help against the shihans I've met.
(Forgive me for the rant) A side note on this topic: there is nothing I hate more than guys who judge my skill as an aikidoist by my lack of muscle strength. When we have guests or I go to a seminar, there's always that guy who won't give me an honest attack or really throw me. So what if I happen to be a 5'8 blond college student? Steam was coming out of my ears when I heard a sempai at a seminar tell his kohai that his technique should be smoother and gentler with me...because I'm a woman. Grrrr.