It's either a sixtel (5.17 gal) or a slim 1/4 (7.75 gal). Looks more like a sixtel to me, and I think Full Sail uses mostly 1/2bbl and 1/6bbl kegs. It should have the volume stamped on the top, usually closer to the edge, although it could be under the label.
I'm trying to find some porn filmed by a dog wearing a GoPro. - SharonaZamboni