critisizing is correct if you are measuring a critter. Although it may not be found in the dictionary, it is a southern term in the U.S.A. used to describe the size of a critter when ordinary instruments are not available. Terms such as " bigger than a skeeter", "longer than my arm", "as thick as your thigh" are but a few terms used. Length can be described spreading ones arms horizontally to the approximate and state "it was this long". These are only a few examples of critisizing.

Though not in the dictionary either, I'd have thought that "crittersize, verb, transitive, is a more accurate word to describe the process you have described above that is used to measure creatures (or critters) in the special circumstances you've kindly detailed for us. As I've done so in the past, I reckon I'm a crittersizer.
In contrast, critisise (or critisize), verb transitive, might very well be used by a critic to criticise (or criticize) a creature (or a critter), though it's not in the dictionary!