Look on the rim on the flat where the valve stem comes through. The wheel size is stamped on that flat somewhere around the circumference. Polaris doesn't make a 12" wheel in a 7" width. It is either a 6" or an 8".
Wheel width is measured where the tire bead sits against the rim. A 6" wheel is usually around 7" wide because of the metal thickness and the roll put on the edge of the metal. Confusing yes but that is the way they decided to do it.