[Smitty]

I usually use schedule 40 in 3/4" or 1" dia. I pick it up at Home Depot or Lowe's. I would let the availability of the required fittings regulate the size. Like the corner pieces that bring everything together. And caps for the bottom or the legs. If you have a long bed I would suggest getting a couple of "t" fittings so as to put a support in the middle of the frame. Since pvc is inexpensive and I would guess that at times the snow may be pretty substantial you would want good support. Might even go for a little over kill and instead of just a cross piece in the middle. Incorporate a cross front to back, side to side in the middle of the outer frame to aid it in supporting any weight that may accumulate. Get the pvc with the thickest wall to have the most support.