Airfarewatchdog.com has a great chart with the carry-on size limits of many airlines. Here is the link. Carry-on Policies

We have found that some airports have their own carry-on bag restrictions, checking the size of bags before being permitted through security. Even though Southwest allows 24 inch carry-on bags, I suggest luggage no larger than 21 or 22 inches depending on the airlines you fly. I am looking for a new 22 inch carry-on with spinner wheels and a remover suiter. Any suggestions?