An APC sensor size is usually 23.5 x 15.7 mm or 23.6 x15.6) But the baffle surrounding the sensor (used to mount to camera body) varies in its dimensions. So while you may have one body with enough room surrounding the sensor to accommodate at 24mm swab, a baffle with less surrounding space may only accommodate a 20mm swab, etc.
Our Swab size for the #3 is 24mm, or roughly 1/2mm too wide but our 17mm swab (#2) is 1.3 mm too small. Our 20mm (#1) is 3.5mm too small.
I prefer the swab size closest to the actual sensor size to get the maximum sensor coverage possible. In many cases, depending on camera maker, the best choice may be using a swab which is just a fraction of a millimeter too large to fit perfectly straight along a sensor edge. In this case, you'll have a choice of using a smaller swab or using the larger one, but "tilting" the swab such that it does not fit perfectly up/ down or left / right. Usually, the tiny area left uncleaned is outside the image forming area. You will leave a small triangle untouched, so on the way back, you have the other side of the swab angled and both ends get covered, leaving a very tiny triangle with its point in the center of the very edge on either side.