On standard 73* quill stems most manufacturers just measure across the extension center to center from the stem bolt to the middle of the clamp.

Cinelli, on the other hand, measures perpendicular to the quill - resulting in an "measurement" that's indicated about 5mm shorter than as measured by other brands.

Your Schwinn should use a standard 22.2mm. You need to match the clamp size to your handlebar. Most [used w/ quill] are 25.4mm (1") or 26.0mm, but many Cinelli are 26.4mm. Don't try to use the wrong size. You can often make it work, but why?