WordNet sense 1 (accepted or approved instance or example of a quantity or quality against which others are judged or measured or compared):

standard of measurement

WordNet sense 2 (diameter of a tube or gun barrel):

bore, caliber, calibre

WordNet sense 3 (a measuring instrument for measuring and indicating a quantity such as the thickness of wire or the amount of rain etc.):

gage

WordNet sense 4 (judge tentatively or form an estimate of (quantities or time)):

estimate, guess, judge, approximate, approximate, estimate