You and United must follow USDA regulations on the size of your pet's kennel. The kennel must be sturdy, properly ventilated, and large enough for your pet to stand, lie down, and turn around freely. The kennel must close securely with a mechanism that requires no special tools to operate.
Prescribed kennels are available at most airlines for a fee.
When in doubt, check with United, because the USDA assigns full responsibility for accepting the proper kennel to the airline. If any food is necessary, an ample supply should be attached in a bag to the outside of the kennel. There must be a one-inch spacer bar around the kennel.