Grab a measuring tape if you have one handy, or a string if you don't, and wrap it around your wrist. Depending on the measurement, your size is as follows:
*If you don't have a measuring tape or ruler handy, you can always use a dollar bill to approximate. Every US dollar bill (regardless of whether it's $1, $5, $10, etc.) is 6.14 inches long. Wrap one around your wrist. If there's overlap, you're an extra small; If the dollar closes end-to-end neatly, you're a small; If you're still seeing just a smidge of your wrist between the edges of the bill (say a pinky's width or less), you're a medium; If you're seeing more like a thumb's width of your wrist, you're a large; and if it's more than that, you're an extra large.