Fraction of an inch are preferred i.e. 5/8ths, 3/4ths, 7/8ths etc. Decimals if you have to i.e. .58", .75" etc. a cheap set of calipers can be had for a couple bucks, I get mine @ Harbor Freight. If you don't know how to read them just google it, you'll catch on in no time You can place the marble beside a tape measure for a measurement & eyeball it if you have to, not as accurate as calipers but close enough.
LINK :: https://www.harborfreight.com/6-in-utility-caliper-63664.html