Great question. Here’s a totally honest answer....I would never taken the measurements, ESPECIALLY on a cat, at the word of the client. I don’t mean to sound like a jerk or anything but measurements can be tricky and forms are expensive, so even tho I would be given a measurement I still would go through the process of measuring the cat several times before moving forward on form ordering..
To answer your question tho more specifically, there are several measurements that are needed on a cat, or any full mount for that matter. Nose to eye (forward corner of the eye to tip of the nose), circumference (at its greatest point in the belly), length (tip of the nose to the BASE, not tip, of the tail), span between ears (not always needed), and tip of nose to base of skull (also sometimes not required). Keep in mind tho, just as with caping these take practice and time to get right and can significantly change to be incorrect if the cat is not positioned correctly, either in unskinned and skinned status.
No matter what pose or form your taxidermist uses there’s usually always significant modifications that have to be done to the form which is very time consuming as well as repetitive....hence why full mounts are $$$$$.
Hope this helped, good luck!
Joel-BRT