I'd agree, by the time the KOTOR games are set in Ossus is pretty much a graveyard after it was pummeled by Exar Kun. But there would be the ruins of the old academy and ancient jedi library there. It is basically like the Lightsided Korriban. I think it would be a good planet to visit.
As for the concept of bigger planets, I personally get incredibly sick of a planet after too much time on it. I personally feel that Korriban was a suitable length for the context of the planet, it served its purpose and if it was any longer I feel it would have been pushing it. Storyline wise told us what happened to Lonna Vash, have us encounter Sion once more and have him put doubt in our character's mind about Kreia, which would similarly be reinforced by the vision of her in the cave.
Nar Shadaa was borderline to the point of getting tedious, Dantooine was fine in my opinion, it could have been a bit longer. Onderon/Dxun were perfect in length for me.
Maybe you could find some helpful info there about the sith