When it comes to Allen wrenches, there are two main size categories: English and metric. The size of an Allen wrench is determined by the width of its blade, which is measured in inches or millimeters. The most common sizes are 1/16, 3/32, 1/8, 5/32, 3/16, 7/32, 1/4, and 5/16. There are also larger sizes available, but they are less commonly used.
To measure the width of an Allen wrench's blade, you will need a micrometer. Place the micrometer's jaws around the blade so that it is snug but not too tight. Then, read the measurement on the micrometer's sleeve. This will give you the width of the blade in either inches or millimeters.
Keep in mind that there is usually some variance between different brands of Allen wrenches, so make sure to use the same brand when measuring for consistency. Also, be aware that some wrenches may be slightly undersized or oversized due to manufacturing tolerances. However, as long as you're using a consistent brand and checking with a micrometer, you should be able to get fairly accurate measurements.