What's a Reasonable Breast Size for a Petite Woman?

At 5'1" and 105 lbs, you are quite petite.  It would be reasonable for you to be a B or a C cup.  During breast reduction surgery, the circulation to the breast skin and nipple must be kept intact when the breast tissue is removed.  This means that there is a limit to how small your surgeon can make your breasts with a reduction.  You should discuss what size you'd like to be with your surgeon beforehand.

Predicting a cup size after a reduction is quite difficult... and if you can find a surgeon that will guarantee a size, I would suggest you get that in writing so that you won't be responsible for any revisions in the future.  The bottom line, if your breasts are the source of your symptoms, you will be delighted with your results.  If your happiness is dependent on achieving a certain cup size, that could be more difficult to achieve and you should expect the possibility of a revision.

As for a B cup on your frame, that would be a personal choice/decision.  Your breast could be the volume of a B cup but it may not have the shape you would be happy with. 

I advise my patients that they have 3 choices in this process when having a reduction... for me to be as conservative as I can (remove the minimums to quality for insurance coverage), to do the procedure as I do (will meet minimums and probably leave you with a small D or generous C cup) or to be as aggressive as I can and leave the smallest mound that I can (results in small C or even a B cup).  This choice can sometimes influence the technique (incisions used) in the reduction procedure.

Good luck with your surgery.