wasn't really thinking about breasts, I meant in general
Oops. Sorry about that.
Selsey Seelowe sells an animation override that takes advantage of a bug to stretch the avatar so that it looks like it's taller than it really is, about 12' tall, though it won't change your interactions with the rest of SL--you'll fit through doors as you did before activating it.
Also, there is Cyanide Ember's BIGAVS system, which also takes advantage of that bug. The first version had you riding a vehicle, and let you have a 76' avatar. The second does not use a vehicle, and has 20' and 40' versions--but overall it's a major improvement over the first version.
Giantess avatars have the problems that prim breasts bring when it comes to clothing, in spades. That said, a friend has one, I think the BIGAVS setup, and she looks quite good at her height (and with her gigantic bosom; remember the photo on which I was a white speck? That was her bosom on which I rested).
Anyway, whichever of these you use, the growth is not continuous; you won't go through all the intermediate heights between your starting and ending height. So in that sense you can't grow or shrink. You can make yourself taller or shorter all at once.