"Tettona" means a woman who has big boobs. It is not a very offensive word as a real insult, since it depends on the manner in which you're saying it. But it is not a good word, as well....
There is no woman who would like to be called like this. I suppose.
If you want to say that to a girl in a more polite way you can say <em>"Sei molto formosa"</em>.