Final Fantasy XII is pretty damn long, though. So are Tales of Symphonia and the Rune Factories.
Seconding the Chrono Trigger rec. Also, the best non-port RPG on the DS is The World Ends With You. It's really unique and has a lot of great things about it, though the gameplay can get pretty hectic.
Paper Mario: The Thousand Year Door is also great, but I can't remember how long it is.