There's an unwritten rule in SL, that you can't be any bigger than what you can carry above the ground. Janeen found some platform boots and with a little work this is what can be achieved size wise...
I don't know if it's an unwritten rule, but I certainly follow it. Alas, attachments are marked as "nonphysical"--that makes them violate some of the rules that matter follows, notably the one about two objects not being able to take up the same space at the same time. That makes for loss of verisimilitude as sufficiently large breasts go underground instead of resting on the ground and flattening out where they touch. Hug a well-endowed woman in SL and you'll walk through her breasts to get to her.
So, as Ginger says, to have
really large breasts, you have to go out of your way to avoid that effect. There are extreme platform boots (a store called Curio Obscura sells them), or an "animation override" that takes advantage of a bug in SL to stretch your avatar until you're about twelve feet tall; that helps.
Remember those "megaprims" I mentioned earlier? Today a friend pointed me at a photo that shows that someone out there is using megaprims to make prim breasts. It's very impressive. There's a speck at the rear of the breasts that, after you stare at it for a while, you recognize is the head of the woman to whom those breasts belong. If I were to guess I'd estimate that they're somewhere around twenty or thirty meters across, or more.