I'm 6'1", and not far off 400lbs, and have recently begun studying Aikido. Less than 10 hours of training under my (so far non-existant) belt, and the only problems I'm encountering are with my rolls, predominantly due to a far weaker left arm/shoulder than my right, rather than any hindrance due to my size or weight.
In fact, the only problems I actually seem to encounter are during warm-ups, where I can't stretch as much as the other people, because my stomach gets in the way