Hi! I'm hoping others will chime in as well, because I'm relatively new to this. I'm rather large chested as well (30DDD in my fave brand). I'm still working out how to get the flattest appearance I can, but thus far I have had good luck with the Tri-Top binder in small from Underworks. I also got the Ultimate Chest binder tank, but it is too long for me, and rolls up uncomfortably over my hips every time I move.
You may need to practice a bit, in order to find out how best to move your breast tissue around to flatten beneath the top. I am struggling with that currently, but it can be done.
Good luck - I'm curious to hear what others have to say on the topic, since I'm still just learning how to work with a binder.
Convention Line-Up: Dragon*Con 2015
1950s Celestia (MLP:FiM), Undead Death Knight Starter Gear (WoW),
Lorna Dane Serval Industries (X-Factor), Adult Loki (Loki: Agent of Asgard), 10th Doctor (Doctor Who)