I don't know what size he uses, but I do know Ben loves lead solder. It melts cooler and flows better than most lead-free stuff so its great for prototyping and anything where you might have to remove something. He jokes that he bought a lifetime supply of leaded solder awhile back.
Go to radio shack and get some 60/40 solder. It's a good general purpose solder. The stuff at home depot is most likely for soldering copper pipes together.
