Are there any pieces that look like trusses? Those will dictate the width of the building and the pitch of the roof. The trusses should be very easy to pick out, since they'll be quite heavy, possibly triangular or trapezoidal in shape, and will have ends that aren't square. If you can't find any trusses, then either they're missing or this isn't any kind of building with which I'm familiar. Even a flat roof will need some sort of beams to support its weight.
You should also be able to pick out the purlins and girts. Both will be lighter gauge material. Girts are the horizontal beams on the sides and ends to which the sheeting attaches; they usually have a "C" cross section. The purlins are on the roof and support the roofing; these typically have an "S" section.
Ultimately, you're going to need to figure out how the structural pieces (trusses, purlins and girts) go together in order to size a pad. Trying to figure out dimensions from the sheeting is a waste of time. You don't know which, if any, of the sheets have to be cut, you don't know the dimensions of the doors and other openings.
There should be a lot of loose hardware, too: bolts, screws, brackets, etc. Try to find every last bit of it, because some of it may be difficult to replace.