What size bolt do we need for our gas tank?

What size bolt do we need for our gas tank? We had to buy a new fuel pump for our '98 Silverado truck. We got that on fine but they lost the bolt or screw that is the 'grounding' screw for the gas tank. We have no idea of the size or whatever and we can't put the tank back on til we either find it or get another. Any help would be sooo appreciated, more than you know!