How rough are the roads that access the Granite Peak trail heads. Would a small car do or should I bring a truck or SUV?

Unless the road has changed a small car should be fine.

The north trailheads (E&W Rosebud) are a piece of cake. The south one (Lulu Pass Rd near Cooke City) can be a little rougher but I've always reached it in a 2WD sedan anyway.

I drove to the East Rosebud TH in August of 2011 - the road was very good. I was driving a 4WD pickup, but it would have been passable in just about any vehicle.

The Lulu Pass approach from the S is driveable in all but the lowest-clearance passenger cars. The major obstacles are a handful of culverts across the road made of chunks of rock, but they're merely annoyances.

Beware of large objects while travelling down West Rosebud Creek Road.

Though I bet the south access is snowed over right now. Mystic Lake and E Rosebud probably plowed open. When are you thinking of going?