I just bought a keg today. It's dimensions are 21" high, 15" diameter, and 51" circumference. I'm pretty sure that it's a 13 gallon. It was smaller than the 15 gl but, that one had extra plastic/rubber on the bottom and top.
Picked it up from a scrap yard. Apparently it's been there for 10 years (I wonder what's inside). I'm switching from extract to all grain and it's been difficult to find kegs. It's aluminum which isn't ideal but it was only 30 bucks. The guy also has some of the more barrel shaped ones in stainless but they also seemed kinda small.