personally, I would put "quite pretty" directly between "pretty" and "beautiful." but it's kind of a subjective thing.
Quite pretty stands on it's own, no real way to tell if it truly means beautiful.
lets use common sense.
when people say I'm not quite there yet they mean that they are not almost there.
pretty doesn't mean beautiful. or else there wouldnt be the would pretty.
therefore if they say quite pretty they mean, almost pretty, or yah I guess pretty.
I'm afraid you'll have to consider the source. I attach great meaning to words, and thus try and select them carefully, in order to better express myself. If I were to refer to you as very pretty, then you could be assured that I meant just that, and nothing more, or less. To describe someone as beautiful, is to offer a higher compliment, perhaps only exceeded (in my mind) by the word "gorgeous".