1.
one round, one lap, full circuit
Going around something once, completing a full circle or tour.
2.
one size bigger or smaller
A noticeable difference in size, used to describe something or someone as distinctly larger or smaller than a reference point.
3.
one zodiac cycle (12 years)
A full cycle of the Chinese zodiac, equal to 12 years. Used when describing age differences.
Usage
Sense 2 is very common in everyday speech. 一回り大きい and 一回り小さい are the most frequent collocations. The size difference is approximate and subjective rather than a precise measurement.
Sense 3 specifically refers to the 12-year Chinese zodiac cycle (干支). 一回り上/下 is a common way to describe a 12-year age gap.