Hi! This might be a dumb request, but can someone walk me through on how to measure my stem's clamp size? Thanks!

If you had the required tool (verniers) it'd be obvious.

Got a shifter or something? Put it across the middle of your bars then measure the gap you needed with a ruler.

Or if you only have the stem, I'm thinking you really need verniers.

Or you can tell us if it's a road or MTB stem or whatever, and roughly what size it is.

If there are handlebars in the stem, measure them with calipers. If not, then accurately measuring an open-face stem can be a bit tough since the opening on the stem or face plate will be smaller than the true diameter and there are some very close sizes, like 25.4 and 26.0mm. If the stem is a 31.8, then it's a no-brainer.

What type of stem is it, quill or threadless? Single bolt wrap around handle bar clamp or removable face plate?
And, you are asking about the handlebar clamp, not the steerer clamp, right?

A pic would be handy.

It's fairly easy since there are only a few possibilities. A stem made for threadless headset systems comes in only 2 common sizes 1" an 1-1/8" (there are larger ones but, I assume if you had something exotic you'd know more). You can use a quarter as a gauge; 1" is smaller 1-1/8" is larger.

Handlebar clamp size is more of a pain because there are a number of sizes, some very similar, so the best way to know is by knowing the brand and purpose of the stem. Most (if not all) stems carbon handlebars are 31.6mm in diameter. Stems made for alloy mtb bars are 1" (25.4mm). Road stems for alloy bars may be 1" if from Japan or Taiwan, or 26.0mm which is sort of the road standard. There are a few non standard clamp sizes in use, but they're fairly rare.

wrap a piece of paper around the bar and mark it, to get the circumference,
then do the math to convert that measurement, with a ruler, to the diameter.
https://wiki.answers.com/Q/How_do_you..._into_diameter