If you are measuring the largest section of the shell (usually the very middle for turbos) inside edge, to inside edge- meaning the literal open space- you can get a general idea of the size. Imo a caliper is one of those extras you really only have to buy if you are struggling with measuring, or you feel like it's easier. It won't be exact meausuring with a tape measure, but the crabbies aren't exact either. It can be trial and error, and the larger shells do get pricey... I would know.

I've got 4 in the 1 inch plus range. Unfortunately since there is a shell shortage in the wild it makes the demand higher, and therefore the shells are more expensive. Richards is my fave, but be warned they do take a bit of time to get to you- I'd say 3-4 weeks after processing, shipping and such. That being said, their customer service is great, just make sure to specify your needed sizes in the comments and mention that they are for live crabs. They seriously are great guys there.

I've had to call them before as well.