Bolt size for tensioner pulley

If it keeps getting loose then it is likely the threads it screws into are worn. I would start by getting a new replacement bolt, then apply some blue loctite to it and torque it to 37 Ft Lb.

Hey Steve, I've already tried that and I still can't get the bolt to grab onto anything!

Okay, then the threads must be gone. In that case I would probably install a helicoil or similar in the hole. You can't really go oversized as the hole through the tensioner is a close fit and there isn't room to go up a size. You can usually get the kits as loaner tools from the parts stores, you buy the correct inserts and they "loan" you the drill, tap and installer tool, then when you return them you get the money for the tool back. The bolt goes into the alternator bracket so no issues of drilling anything important. Once installed the helicoil will make the threads better than the OEM ones.