I have multiple "Damaged Screw Remover" bits. I have another instance of the screw in my hand. The bits are designed for specific screw sizes (No. 4 to 7/No. 8 to 10). How do I determine the "No." size for the screw?
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.
There are thread gauges you can use to determine the diameter and threads per inch. For instance the "10" in 10-32 is the diameter, the "32" means it has 32 threads in a length of 1''. The larger the diameter number, the thicker the bolt. If the threaded bolt is stuck in something you can use the gauge to compare the size and that would give a close guess. When using the easy out extractors, I use the largest one that is still smaller than the diameter of the bolt.
Here is a chart of screw sizes
Measure the screw and compare it to the chart.