How to determine the screw size (No.)?
If you have an identical screw, you can probably take it to Home Depot or Lowes and use the thread gauge mounted in the hardware section. (The HD in my area has one, but I'm not sure if all of them do.)

This website also has several printable size gauges for screws, bolts, washers, and nuts.

Otherwise, you can buy one. They're usually labelled as "Screw ID Gauge" or "Thread Gauge." Home Depot has them in the hardware aisle for $2 or $3.