Shell measurement

[Lrborange5]

I have a caliper I just got. I know how to measure and read it. I know to look for oval or round shape.
When looking for bigger shells, how much more should increase the size when looking for new shells?

Is it likely for a crab to choose a round shell when it previous shell was oval?

[hermiwormi]

When looking for shells, I usually get a variety of larger, smaller, and same sized shells. My reason for smaller shells being that my newer crabs chose smaller shells before molting.
Edit: Also if you're unsure, get a variety of shell shapes so that your crab(s) can choose what they want to wear, then you can see what your crab(s) prefer.

5 PPs: Koa, Moja, Siam, Chella, and Ozark.