The current study examined the effects of grade level and object size on the ability to estimate the measurement of objects. Fifth- (n = 198) and sixth-grade (n = 208) students and freshman undergraduate students (n = 71) from Taipei city, Taiwan, participated in the study. The findings indicated a tendency for measurement estimation ability to increase with grade level. Overall, estimation performance was most accurate with medium-sized to-be-estimated objects (TBEOs), while estimates of small-sized TBEOs were more accurate than those of large-sized TBEOs. A tendency to underestimate the measurements of large-sized TBEOs was observed in all groups. Those with good estimation abilities showed a preference for using body parts and convenient objects as references. Moreover, the integration of measurement units constructed from previous experience and eyeballing was an essential skill used by good estimators. Suggestions for measurement estimation instruction are discussed.