Tires come in all different sizes and widths.
Rim sizes are typically 26", 24" and 20".
Cruisers 26", musclebikes 20".
Widths typically used for cruisers and kustoms...
Ballooner: 2.125"
Middleweight: 1.75" or *Schwinns 1-3/4"
Newer tires can even be 3.0" or wider.
*Note the Schwinn 26" x 1-3/4" tires require a Schwinn S7 rim and will not work on a standard 26" rim.