I'd actually recommend larger tubes with thinner walls rather than smaller diameter tubes with thicker walls.
Obviously there's a tradeoff between stiffness/weight/toughness. A small diameter (relatively) thick-walled tube that weighs the same as a large diameter thin-walled tube will have more ding-resistance and toughness, but less stiffness.
...and I wouldn't vary radically from existing designs.
NRS tubes are 1 1/4" SCH40, or 1.380 ID with a or 0.140 in wall. OD is about 1.66"
Speedrail uses 1 1/2" SCH40, which is 1.61ID, 0.145" wall, and 1.9" OD.
You could consider 1" Sch20, which is 1.097ID/1.315OD/0.109 in wall...I wouldn't go much thinner/smaller than that to start with.
Or go a lot smaller, break it, then start stepping up! haha
since other manufacturers only sell large tubes. Is there a manufacturer who does smaller tubes?
Are you building this yourself or having it fabricated for you?
Search around here, there are several custom frame builders whose names frequently pop up. Recretec, Mad Cow, Eddyline Welding, Neff's Whitewater Customs and Riverboat Works are several of them. Neff's and RBW are Buzz sponsors. If you're having one built, pick a vendor, and work with them to source smaller tubing and a lighter frame. I'm guessing they've done it before.