True measurements of the mesasoma are usually given by most, but other measurements including other appendages are sometimes given also, but usually noted so. Ie) My scorpion is 3.5 inches head to tail.
I don't see including tail size useful because it overhangs the body, so it doesn't add to the scorpions general size under normal conditions. The way I picture it is that I am only 5 foot 10 inches, but with my arms reaching up, I could say I am almost 7 feet tall.
Which way is the "correct" way, who knows. I'd just say, it's mesasoma is X inches, or the scorpion is X inches head to tail. Nomenclature is everywhere, especiaaly in an under reasearched topic like this. Once you start deeper research you learn not all authors refer to body parts and actions in the same terms, somewhat confusing.
If anyone knows the way most scientists would record measurements for research, that would be nice to know.
Measure the scorpion from the mouth parts (chelicerae) to the tip of the sting (aculeus). This is if you want an overall 'general' size measurement. Measuring the body (mesosoma) by itself is not particularly useful as it varies so much according to many factors, such as pre-molt (ecdysis), food consumption and especially for taxonomists the fact that when preserved they often stretch in alcohol. The tail (metasoma) is a good measurement as it doesn't vary other than from instar to instar and between sexes etc. So, the metasoma is really a very useful measurement, but in your case just do a snout - tip of sting measurement. Try to keep the metasoma straight.