One of the things you might want in a blue grass guitar is a wider neck. It seems to me that runnin' all aorund the fretboard, flat pickin' all those notes is made a bit easier when you have more room on the neck.

The sound I most often associate with bluegrass is that of the Martin Dreadnought. Santa Cruz makes a "Tony Rice" model that has that classic sound, with a bit more highs (somehow - a crisper sound). I love the sound, and it plays well - costs quite a bit, though. I've never seen bluegrass played with a jumbo, but think it would sound great. The Larrivee jumbo has a near perfect balance, a heavy bottom and it's highs are crisp, and loud? My friends call mine a cannon! It is loud.

Of course you probably can't go wrong with a good Martin, and there're a lot 'em to choose from. Best of luck!