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Is there a kennel bigger than the XL?

Yup, for International Air Transport Association (IATA) kennels, there is 'giant,' which is 48" long. Good luck getting that one on any short-range jet, though, as the standard cargo door for most 737s is only JUST large enough to fit an XL VariKennel (45"w x 34"h).

FWIW, Turbo's home kennel is silly-large and not airline approved: 48"h x 37"w x 54"d. It is big enough so that I can't fit it through a standard house door without disassembly!

Most carriers have a cargo shipping guide online... here's Uniteds:
http://www.unitedcargo.com/shipping/container_aircraft.jsp?name=747-400&type=fleet

Randii