Personally, I think the "ideal" size depends on the person. Some people want larger cups, some people don't.
Dani is going back and forth right now. Some days she says she wants a "larger" chest, but not as big as me and some days she says she'd really like to be as big as I am. *shrug* It doesn't matter to me really, as long as she's happy. Right now I've been telling her that if she doesn't like what hormones give her, then we can fix it later. However, she has to be sure, because she has to haul 'em around 😛
My current size is my "ideal" size. I've actually been asked if I was going to get reduction surgery. And my answer is "Nope! I *like* my chest. A lot of women spend a lot of money to have what I do for free!"
Also, big breasts do not automatically equal back problems. I've not had any problems with my back at all. The only time I had any pain was when I was wearing improperly fitted bras. Once I got good bras, the pain vanished...pretty much instantly too. It was amazing.