3/4" diameter by 15" long Eyebolt. The 15" does not include the eye, but leave enough space to get the nut/washer on and I like to see 1/2" or so sticking out because you can tell visually from a distance that the nut is not just barely holding on.

This bolt is designed for 100% tension loading with a strength of 17,000 lbs. However, if the load is not in tension, the strength can be drastically reduced.