Best idea is to actually measure one of the buttons using a ruler or tape measure and state that size in the ad. If you can't show the back, just try to get a look at the back from the side and report what you see. To measure buttons, just measure straight across the top and report that size in inches or increments of inches. Some measure in millimeters to be exact, that's not really necessary with pretty buttons although uniform button sellers often report millimeter sizes. Since many carded buttons are not particularly collectible (too modern or common), buyers are usually individuals who want to use those buttons in crafts, sewing projects, etc. They need to know the actual size of the buttons you are offering rather than guess. Also, if you are measuring a square button, measure it from point to point on the diagonal and report that size.