Hi Jackie,
I have been blessed in a number of way, one being I have my own set of medium 'B's'. I do however associate with many different TG communities in Sydney, and one thing I have noticed particularly is that women that need to use forms, generally buy too large a size, naturally drawing some unwanted attention.
My theory is that, when we look down on our breasts, we see quite a different perspective to what others would see. So what might look absolutely natural from another persons perspective looking head on at us, may look somewhat small to what we would see, looking down on them.
Therefore, if you have a current band size of 35", I would suggest not going anything larger than 38" round the bust, approx equal to a 'C' cup, in order to maintain a relative feminine figure, avoiding the "Dolly Pardon" look. Do your best to find a store that will give you a personal fitting. That way, you'll receive there professional assistance in the bargain.
You may wish to experiment first, using balloons and bird seed. That way you can make breast sizes 1", 2", 3", and 4" to see which works best for your overall figure. You may need 2 bras for the exercise. A 'B' cup and a 'C' cup. Make sure you wear the same top each time, and you may wish to photograph from several angles to give you a better overall perspective of how people in the street will perceive you. Dolly Pardon or Miss Twiggy, hopefully something in between.
Enjoy and have fun with it. It's you. Hopefully HRT will do the right thing by you. Keep in touch.
Be safe, well and happy.
Lotsa huggs
Catherine