8" is my go-to size. The jaws are a little deeper than a 6", my clumsy hands have an easier time with hanging onto it, and it's not too large to fit my workspaces. And Fowler makes (or used to at least, can't say about recent mfg.) pretty decent instruments. Or rather has them made might be more accurate in most cases.
i bought a cheap 4-inch caliper to carry in my shooting kit and find it hard to use. next time I think i will stick with 6-inch models. the 4 inch for some reason is just harder to use. just awkward maybe i am too used to the 6-inch size