Make sure to measure your pet while standing from the base of his tail till the tip of his nose and add 4 inches to this measurement for the length.
For the height, it might be tricky so you could see where the top of the head of your pet is at compared to your legs and then you can measure your legs instead to determine how tall the crate should be. It is recommended to add 3 inches to that value.
You should also consider measuring the weight as some crates have a maximum weight limit.
*Snub-nose breeds require one crate size larger than the indicated measurements.
Measure while pet is standing, from base of tail till tip of nose and add 4 inches.
Measure from top of head till the floor. If proven difficult, see where the pet is touching your leg while standing and measure your leg instead. Add 3 inches.
Use weight as a guidelines also as some crates have maximum weight allowance
A visual guide on how to measure your pet for the travel crate size