The size of a screw is the size that would have been 100% mesh so your 1/4 inch screw will fit into a 1/4 inch hole with a small gap. There are 3 common standards for machine screws.

I've never seen 4-40 thread standoffs (that is the size of the hold down screws for DVI, VGA, serial, etc.), but definitely have seen both 6-32 (aka hard disc screws) and M3 (aka CD-ROM screws ...