The heads of the bolts are 9/16. Thread size is the same as the leg bolts, so if you don't have any, take one along to get the right thread size. The head bolt is about an inch longer than the leg bolt, with a washer. There will be four of them.
Best way to put the head on is to line up the square cutout in the bottom of the head with the square cutout in the top of the body. Make sure you pull the line cord out first and get it in it's slot, or you'll be cussing up a storm after you have it bolted in. Then get some bolts in and adjust as necessary until all four will freely turn in their sockets. Make the backs of the head and body are flush, then tighten them down. Don't forget to connect the ground strap from the body to the ground strap of the head (one screw).