For comfort, standard long action calibers should be a minimum of 8” calipers for the body comparator, I don’t have a magnum long action, so a 12” maybe better idk. 6” works fine for short action but I prefer 8” anymore……I’ve got 8” for everything I have.
Also, I bought most of mine on eBay used……$250-$300 calipers for less than $115. Now you need to know that there are fake mitutoyos out there, do your research on fake vs real mitutoyos.