Do the cap shingles that install on top of ridge vents come in certain sizes?

The most common size is 12-inch wide. Exposures can vary but the standard width is 12 inches. There are also 9-inch wide. If the contractor is using an actual cap shingle, there is no need to cut or trim the shingle. If the contractor is using a 3-tab shingle to be the cap shingle, he'll have to cut those. A fairly inexpensive way to buy cap shingles is to actually purchase 3-tab shingles and then cut or trim them to become 12-inches wide.