Then there is the thread type, there is fine and course threads.
What you want is probably 1/4" long and probably in the 8 or 10 or 12 size. They are probably fine thread.
If you have a screw that fits take it to Home Depot or Lowe's or a hardware store. In the hardware aisle, where the screws and nails are you should find a metal plate with lots of holes for different size screws. Try you screw until you find the one that it fits. That will tell you the size and thread.
If you are lucky you may find some one to help you.
If you do not have a screw, purchase a packet of each size, both fine and course, that looks about right.