Just rambling here, but I think it's important to "prepare for the worst" all the time. (I work in software testing QA... sometimes, preparing for a nuclear blast is inadequate!)
To translate that into real-world use for you?
I'd say, larger is better for forms, as long as it's not ridiculous.
Measure your chest under the breasts, get the real measurement. Now, for a genetic male, you DO NOT add 3" to that (and for women over 40", BTW - but women tend to taper more in the ribs, so unless you're both very young, very petite, and have been training the ribs? Follow the "guy" version, and you can pursue rib training in parallel, which is a separate topic.)
So, if you have female relatives with D cups? Assume you'll get at least D cups.
Then, go to the Breast Form Store - specifically, to their site - and scout out what size you need for various manufacturers. Make it a D cup, say, and then go looking for a cheap provider. Caveat: you WILL want silicone. NOTE: You CAN make your own, there are videos and tutorials around. So make sure you do a good search, and value time vs. money vs materials, and worry more about WEIGHT than Realism.
Why weight? Realism is incidental for this. But a tough, durable form that can be glued on? One that bounces fairly realistically and has proper (even excess) weight? THAT is what you need - not an Amoena feather-weight, thought they're supposedly good forms. You want the weight, so that when you're experimenting, you can try odd things, like jumping, a trampoline, working out, running, driving (Seat belt + road bumps, two events), and doing things WITH a bra all the time - AND, what happens when you take the bra off...?
For example: You come home with your full-grown, mature girls. You won the genetic lottery, and have D's (maybe larger).
You come in, strip off the shirt, pop your bra band, and the girls drop 5" down your chest, so you re-fasten the bra.... :-(
And when you go to sleep, they bounce around too much, which hurts, and you need to sleep in a bra....
See the issues jumping out? Also, see why I'm suggesting you go large, though not ridiculous?
Try yoga - Downward-facing Dog. Can you breathe?
Also, thinking long-term, what do you want in future relationships? Are you looking for men? Women? A little of both? Are you looking at worldly success? (I'm an AVP at a BIG, billion-dollar a DAY company - if I weren't a fat SOB, my boobs might cost me the job. As it is, they fade away into the rest of me... But if they were really D-cups, as the tape suggests, I might be working at the Starbucks downstairs, JUST to have an income...)
Also, the "worldly success" works FOR men, and AGAINST women, in many ways. If you're presenting male (with or without boobs), you want the position. E.G.: Successful guy with boobs = gyno, unlucky, but "stuff" happens. BUT, Low-level guy with boobs? Loser. Personally, I think it's BS, but people infer a lot. Women talk about this a lot WRT breast size: Blonde or big boobs? Bimbo, dumb, easy to take advantage of, Easy. Doesn't have to be real, people act certain ways, perceive things based on their own biases.
This is a life thing, definitely need to consider the impacts, plural.
The thing I see is, from anecdotal evidence and some experiments?
Most people won't notice breasts on a guy, or won't say anything. If you're obese, it'll be noticed less; it will also be less noticeable, and less feminine.
You can have relationships, with men or women, and there was a book by Norah Vincent, a lesbian, called "self-made man," where she tried dating (among other things, in her attempt to confirm "male privilege"). She found that the women were willing to continue the relationship, even after she told them the truth about being a woman. This was a second date or so - no "sunk cost" to be concerned about. But finding that woman might take some time.
If you go the woman route, even if you keep the original equipment, you've got a different standard to deal with, and having big breasts may get in the way... Also, they DO get in the way, even small ones! Mine have the projection of a B cup, maybe, but that's enough to interfere with carrying something on the chest, or a bench press, or pushups, or hugs, or even driving - seat belt and wheel both. Coats don't fit right, most clothing which will accommodate breasts is meant for women - so, darted shirts/tops, which ... don't work for a man. ;-)
If possible, try before you buy. Try dropping your testosterone down, too. Spearmint tea, Saw Palmetto, try bicalutamide (interferes with the actions of testosterone by blocking the receptors; this could mean higher DiHydroTestosterone, DHT, which means higher facial hair and male pattern baldness risks), spirotone, aldactone, finasteride, dutasteride. Research the dosages and sources, and be aware there are risks. (Google "Post-finasteride syndrome," you can permanently lose some male sensitivity.)
then there's the effect on the rest of the body, too... Muscle mass can stay, while strength goes. (Major problem for me, I need to be strong!) Swimming messes up the shoulders a LOT, apparently, leaving them weaker than you'd think - they're TOO loose, too prone to injury. Weight training would fix it, but.... Manly weights. :-P then there's the Trapezius, which - if you've been doing butterfly - well, that's a difficult visual to overcome, and bra straps slide down easily.... (I know from experience!)
Plan more, experiment first. You could easily do water balloons at first, cheap and easy and adjustable. Get the idea of things on your chest. Then go to a silicone DIY, gives more weight, could probably be stuck to the chest for extended wear.
Then maybe the actual costly forms, and again - stick 'em on.
If, like me, you are in love with the weight, feel, idea, the curves, the ability to touch them...? Then it makes sense to see a gender therapist and start serious plans. (I haven't done this yet, as I'm paranoid about the existing abuses of power by our loving government.... you don't need "X" until they tell you you can't have it, "for your own good." I don't need to be protected against myself, I need to defend myself from the lunatics next door. Whether they should "allahu ackbar" or "Freeze! Police!" is a matter of degree; I can shoot the former and worry later; but, if I wiggle, the latter can kill me with impunity. But I'll leave politics aside from here on.)
Make haste slowly. I found online pharma and I jumped in. I'm of two minds about it: 1, I wish I was bigger, and had done it all when younger and pretty. 2, I wish I could be "normal," whatever that is. ;-)
But I love having breasts, I just wish they were more sensitive. I pass as male, I have a ... Female, sort of, in my life (Don't ask, she's heavier than I am, and a win-at-all-costs type), and I LOVE putting on a bra and admiring the visible curves. If I could lick my own nipples, I would. I'm looking into a Brava/Noogle/Etc, and a breast pump, in the next year, and hopefully can upgrade a whole lot of things in my life. Including the necessary therapy (from multiple causes, not just this one.)
Take a look down each side of the road and think it over, and experiment. And let us know, success stories are always great! :-)
-Jean
