if you are going to use telegraph poles for the uprights , you will also need some 4"x2" , for the cross members. These will do for spans of upto about 10' after that you would really need 6"x2", or maybe 4"x3" . It really depends on what you can get hold of . If you want a small open field shelter type thing, which is just 2 sides a back and a roof, then 10'x10' may be big enough , if not just make it longer 10'x20' or whatever suits. a 10'x10' you would need 4 tele. poles and 3 10' x4"x2" for each of the 3 sides , and 3 for the roof. The front would remain open and you can fit some sort of gate if you want to close animals in .The poles would need digging in about 2' ish at the corners of the shed, make the size to fit what type of covering you are going to use , so there is minimum waste . once you have got the pole in and they are upright !!! and square, you can fit the 4"x2" side and back members. Fit the bottom one about 1' from the floor and then one in the middle and one at the top on the 3 sides. Use either 4"-5" coach screws or 6" nails to fix them on with . The front of the shed should be taller than the back , say 8' at the front and 7'-7'6" at the back . The top 2 side members should go on at an angle, 4" from the top of the back pole to 4" from the top of the front pole on the outside, then you can fit the roof beams from side to side ontop of the side top members. You put one top beam at the front, one in the middle and one at the back. You can now cover the framework with corrugated iron , if you going to use something else you may need to add extra timbers to fit it on to , but this setup would do for corrugated iron or even box section steel sheeting and even plywood or flake board .
I can never tell if what I have written makes sence , so hope you can understand what I mean !!
cheers anyway
Russ