Its just a standard socket on the tensioner pulley to remove the belt. 15mm I believe. To release tension, turn it counter-clockwise (towards passenger fender well). All I had was a 3/8" drive socket with a stubby handle when I did mine, so I put a 2' long piece of pipe over the handle and used that as a cheater bar. Plenty of leverage to release tension with one hand and get belt on/off with the other.
If you're removing the tension pulley (or idler pulley for that matter) from the arm, you'll need a t50 torx attatched to a 1/2drive socket IIRC.
My memory is a little fuzzy, but one of the tension/idler pulleys might be reverse threaded. Hopefully someone else can chime in on that.