The present study investigated the effect of the physical estimation environment and the size of the to-be-estimated (TBE) objects on the use of measurement units and length estimation performance. Following a between-subjects design, forty young adults were asked to estimate the length of nine pieces of ribbon presented in two different conditions: an External Measurement condition, wherein several external measurement units were present or a No External Measurement condition in which such external measurement units were absent. The results indicated that the availability of external measurement units affected the type of measurement units that were applied as well as their frequency of use but not the estimation performance. Furthermore, it was found that participants adapted the size of the measurement units they used to the size of the TBE objects. At the same time, estimation accuracy was correlated positively with the frequency of use of external measurement units and negatively with the use of idiosyncratic units.