I am currently putting partitions in my new 42x60. 16.5' eaves....about 21.5 inside at the peak.
I decided to use lightweight steel studs -- can get in whatever length you need, and are straight, and will stay straight, unlike trying to find good 20+ wood nowadays. All one piece, so you don't have a hinge point as if you were stacking 2 wood frame wall sections on top of each other as some do.
Wall is real flimsy when studs first go up, but putting on horizontal hat channel at 4' intervals makes it solid as can be.
Little bit of a learning curve, but once you have it figured out, it goes up easy and fast.
Cheapest at a drywall supply house.