Take a trip to Home Depot or a hardware store. I'd bet they are either 1/4" or M6 (metric). 1/4" has a fine and coarse thread available- get both. Just buy one of each size, and maybe one size larger and one size smaller.
See which screw goes in without forcing it, and voila- you'll know the size. Should not be more than 2.00 total, then return the screws and it's free.
See which screw goes in without forcing it, and voila- you'll know the size. Should not be more than 2.00 total, then return the screws and it's free.