Hi Mollie, most wire jewelry uses gauges from 14-gauge (thick) to 28-gauge (thin), with some pieces and techniques using even larger and thinner wire. The higher the number, the thinner the wire; to help me remember, I think about a small tube that I put wire in, and I can fit lots of thin wire (like 28), but not as many thick ones (like 12). Wire jewelry patterns usually choose gauges from all the even-numbered gauges plus 21-gauge.
The wire sold in North America is usually measured according to the American Wire Gauge (AWG), one of a few ways to measure wire. Wikipedia has a lot of information on this measuring system, including how the steps between sizes are calculated and some rules of thumb.
Here on WireJewelry.com, we have a handy page for finding the diameter (width) of the wire in inches and millimeters. Simply go to our Resource Center
and look under "All About Wire" it's called Wire Gauge & Footage Charts
. The first two charts are for converting the weight of wire to footage (because some people are used to ordering wire by the ounce or the pound, and we offer jewelry wire by the foot); the last chart will help you convert AWG to inches and millimeters.
As far as your draw plate, to find what gauge a hole is, I would measure the diameter of the hole with a ruler that has millimeters, a wire gauge, or a caliper. Just use the chart I linked above to convert the millimeters to AWG. Be aware that many draw plates are chamfered, meaning that the hole is wider where you put it in, and tapers to be smaller as the finished size. I hope this helps!