I LOVE massive boobs. Honestly even in real life having a chest with basketball-boobs is awesome.
As for fantasy-wise..
I don't always draw musclegirls with the massive boobs as we can see here but I also like to draw girls really, really pregnant sometimes so their boobs can get even bigger (I won't go linking to one of that type.. I doubt most people here would want to see anyway).

However, the LIMIT for me comes when boobs get so big they go lower than the person's crotch. I just don't like it so much when they get bigger than that.

One thing I might add; I think my personal limits in what is too big have progressed over the years.

When I first started out, I thought breasts like Readyart's and muscles like SyberStyk's were too big. Now I'm rather quite fond of both lol. Anyone else have this problem? Uh... not that it's really a problem...

One of the interesting things about a fetish is how easy it can be to cast a wary eye on variations that are outside your own personal norm.  When I think about what I consider to be some completely outrageous representations, I think "how can that possibly be appealing?".  But then I take a step back and look at how my own basis is far outside the norm, and I say "oh yeah...".  Thus, for questions like this, there are clearly no right or wrong answers.  Not that I feel that other posters where implying that there were absolutes...I just felt it needed saying.

Given that, functional use, as others have stated, is also my key dividing line.  Fantasy breasts can still be quite large, but they should be above the waist and still appear to be supportable.  The beach ball look is ok, but some realism is even better.

Desensitization?  Hell yeah!

When I look at the likes of Melissa Dettwiller and Karen Zaremba I consider them "skinny" in my books.  While an average joe shmo would say "OMG IT'S THE HULK!"

Morons.