Any number online pole barn suppliers, or places like Menards, provide lumber packages. See what lumber dimensions they use. But, I would not use anything less than 4 x 6 posts. Truss spacing of 4 ft is common, but it depends on the truss engineering, snow load, wind load, and so forth.
I have a 14 x 22 pole shed. Used 4 x 4 posts at 8' and 6' spacing. Corner posts could have been 4 x 6 posts. Double girders ("top bands") are 2 x 8 bolted to posts. No trusses, but rafters. 2 x 8 joists span the 14' width, 2 x 6 rafters at a 12/12 pitch are either 24" or 16" OC. (Can't remember offhand).
