I'm not sure there is any specific set of "standard" dimensions for a non-formal sporran. Thompson's "So You're Going to Wear the Kilt" gives dimensions, but they seem a bit too small to me. I recently made a skunk sporran for my future son-in-law that was less wide than the ones I made for my son and I. Scot is thin and not short and wide, so I thought it best to make one that would 'fit' with him. Often, if I'm working with a pelt I let it tell me what size it wants to be—that work's pretty well.