GMH: I'm personally of the opinion that Katahane is godawful. As for why, the romance is pretty shallow, one of the main characters is kind of a dick that people bend over backwards for but doesn't get called on it, inappropriately timed sex scenes, a weak story that is only good during the part that takes place in the past and finally, damn is Coco creepy.
Anyway, as for your other points, you're kinda going to find it difficult to find something that meets all those criteria. For clean VNs, there's Katawa Shoujo
optionally, Higurashi no Naku Koro ni
, Umineko no Naku Koro ni
off the top of my head. Art quality probably disqualifies Tsukihime, Kanon, Clannad and Umineko. Higurashi has an art patch to fix up the horrible crap. Ever 17 is a couple GB in size though and that's pretty standard. Higurashi is a bunch of separate, small VNs that are actually fairly small.
I wouldn't say any of those are horribly depressing but then I'm not really depressed by anything short of Cross Channel
, which is one downer of a story at times. Anyway, I'd probably recommend Higurashi or Ever17 if we're sticking strictly to your criteria, probably favoring Ever17. I'll also throw G-Senjou no Maou
out there. It does have sex scenes and it's larger than you'd like, but it's got good art, good music, isn't depressing and is really good. There aren't many VNs out there less than 1GB.