I believe there are 8 bolts.
two right behind the cab, where the factory roll bar mounts are.
Two right behind that where the frame arches up for the rear axle, right above the front leaf mounts I think.
Then 4 more on the flats of the frame behind the rear wheel opening.
This is to the best of my recollection, I did all the bed stuff like 3 years ago. You'll probably want a breaker bar, space is tight up inside the inside of the outer bed skins, so it'f you've got some deepwell 1/2" drive sockets and a few feet of 1/2" extensions take those, or you'll curse every time you turn the wrench and bang your hand.
As to size, something between 9/16 and 7/8, I can't remember if it's metric or not or the exact size, as mine got cut up and welded back onto the frame.
Edit: I had a longbed, there may be differences in the shortbed, but pobably not much.