hello,
i'm small and i find thats fine for me, and i know tall people who say the same. personally i think its great that there is a mix in aikido of people of different heights, weights and strengths, because at least you'll find out if you're technique works on different people. there's nothing worse than practising with people who are the same size and strength as you all the time, then finding out you've been missing something and your technique won't work so well on someone bigger/smaller or stronger/weaker. plus you'll have an advantage if you have to do front breakfalls over a row of people
incidentally most of the tall martial artists i've met have been thai boxers; i think they do well because of the long legs