Since you're asking here I'm guessing you won't be welding your own joints.
The simple easy, home depot way:
Make 2, 20'x20' structures. You can find tarps that fit and you have more flexibility with how you use them. Also more manageable. I'll give you a rough way to make ONE
flat roof 20'x20' (if you want two follow directions twice). Flat roof means you'll have to keep it from gathering water if rain happens to hit, but they are easier and good for the wind.
Use the biggest pipe you want and that you're willing to lug around. If you're using pipe, 1' schedule 40 (standard), you should be fine. If you want something beefier, make it beefier. I think some home depots will cut and thread the ends of the pipe for you.
Cut 8, 7' lengths for the legs with one side threaded NPT. You'll probably want one leg every 10', so one for each corner and one on each side. Adjust length for the height. The bottom of these do not need threads so you can always make it tall and cut it down easily later.
Cut 7, 10' lengths for the roof of the structure. You'll have 2, 10' lengths along each side (we'll get to the 8th piece soon), connected in the middle to a leg with a T pipe fitting and at the corners with a pipe fitting with a hole on each axis (x, y, z, not sure what those are called)
4 (x,y,z) fittings for the corners
4 T fittings for the mid section of each side
1 union fitting
You want to cut your last 10' length in half and subtract the length of your particular union from one or half its length from both of your ~5' sections. This union will allow you to thread all of your pieces together.
Now you just need to cover it with a tarp and stake it down. and remember all pipes need NPT threads on each end except the legs which only need one end threaded.
I just made this up right now so make sure to double check everything and don't blindly build it, but it should work as is.