I bought some cheapies from Kmart/Target (in Australia) to test... I had a rough idea and used the "cheapies" to confirm (actually still have a couple as they are soooo comfortable, these are the 14B's) Can also get away 12B's (sorry 34B's) depending on brand

As for type, it depends on what you want to wear them under. Personally I go for plunge lines when wearing anything V neck or semi plunge, balconette bras for low necklines, and full or stitch-less for under sweaters and under tees for the latter...
Works for me, Hope that helps
L Katy