More likely 21". I rarely see any thing over 22" in the self propel mowers; unless, it is a commercial unit. If it is like others here you need to remove the rear baffle to get to the belt. Can be fun (not easy) to remove too. These v-belts are usually a 3/8" (3L). Also you could have a broken drive cable.
What is the model number? I can probably check here for the belt size.
