I don't know how much the following will help, as it pertains to AA, but I suspect (as you do) that all USA carriers have similar requirements:
Pets Traveling as Checked Baggage
The maximum size for checked kennels is a series 500 kennel with the following dimensions: 40" long x 27" wide x 30" high. However, this size kennel is not accepted on the Boeing MD-80 (S80). Kennels checked on MD-80s must be able to fit through the cargo door while remaining in an upright position. MD-80 cargo doors are 29" high x 53" wide.
Series 700 kennels are not allowed on any aircraft.
The maximum weight of a checked pet and kennel (combined) cannot exceed 100 lbs.
Collapsible Kennels (those which can fold down flat) can not be accepted due to the risk of collapse during transport.
Pets Traveling as Carryon
The maximum size for cabin pet carriers is 23" long x 13" wide x 9" high.