Female to male: what can I expect from testosterone therapy as far as breast size?

Thanks for the question.  This can vary in my experience.  Some patients can experience a dramatic decrease in breast volume, but I have seen others that have very little response.  Most would suggest giving it a full year to assess the type of response.  After that you can consider TOPS surgery if there is still a substantial amount of breast tissue, excess skin, and larger areolas.  Hope this helps.  
"The above answer has been given without seeing this patient and cannot be substituted for actual in-person examination and medical advice; it is only opinion. Seek in-person treatment with a trained medical professional for appropriate care."

You may expect some small decrease in your breast size after approximately one year of testosterone therapy.