We used a 4 person tent for three adults for 73 days on the Trans America with no complaints other than me complaining about the weight (9 pounds). I think we would have been fine with one of the less skimpy sized three person tents too though.
Personally I don't bother with vestibules for bike touring. All gear that isn't either needed in the tent at night or needed to dress in the morning stays in the panniers, and the panniers stay on the bike. A bit of overhang is nice just so it doesn't rain in, but vestibules just aren't worth the weight (to me).
If the PALM ride is sagged, take a big roomy tent and don't worry too much about what it weighs, within reason.
Last edited by staehpj1; 11-24-09 at 01:16 PM.