Wow, I don't like the sound of having to learn ukemi the hard way! Is that really possible?
Where I used to train there were a lot of people from the building trade, who were very big and beefy. I found them intimidating at first, however for me it felt like part of aikido training to face someone who is obviously your physical superior, and be able to deal with it (both as uke and tori). [the females at the time tended to be alot more experienced and had no problems with them].
I think one difficulty females often face is that they may get thrown on to their chest (if she has big chest/breasts it may be very difficult for her to do a fast break fall without hurting herself and she may feel that it is not so much the power but the speed of the throw which stops her preventing this). - this may also be something which she may not feel obliged to point out to you. [us men have a similar groin problem if your legs happen to close together too quickly!]
Best advice I could give is to get to know her socially, so she trusts you as a person (or just let her get used to you down the dojo). Then eventually when she has the courage, train with her very gently and slowly (and so she can do a roll, rather than a breakfall) - gradually picking up power and pace over the weeks making she that she still has control of her own ukemis.
To me fear of our opponent or a weapon is an intrinsic aspect of aikido which we have to overcome (which is why I like bokken work).