Here are my thoughts, take 'em or leave 'em:
1. The idea of not training with someone due to thier size and weight is absurd. You train to improve yourself, why limit your training by limiting your training partners. The more variety you get in body types the more rounded you will be as a martial artist and better prepared to defend yourself if need be.
2. As far as 'real world' street application is concerned, Aikido is probably not ones' best choice for a martial art. (Wait don't freak out let me explain). I see Aikido as a spiritual "Art" moreso than a practical self defense vehicle. True, those who can do it at a VERY high level can effectively defend themselves on the street, however, I think hour per hour on the mat a person who learns to kick, punch and block a right cross will fare better than the Aikidoka.
I've trained Goshin Budo Jiu Juitsu and currently study Aikido and though I love my Aikido training you can bet I'm throwing low kicks in fight (at least until I'm at that higher level in Aikido).
Additionally, Aikido really doesn't provide for the fight that goes to the ground where most end up. Miss that block/tenkan/lead just a tad and you'r down on the sidewalk trying to figure out how to do Kote Gaeshi from your back with a drunk on top of you.
So in that respect I think Aikido lags behind some other martial arts in terms of practical application on the street.