Determining good estimates for lengths of objects or distances is an important ability for many professions, but also in everyday life. However, many students struggle with estimating lengths. In a study with N = 335 junior high school students, we investigated their skills of estimating the lengths of daily-use objects and their use of estimation strategies. To understand students' mathematical thinking while processing mental measurement, follow-up interviews were conducted with 45 participants. Results revealed students' moderate length estimation skills. Their performance did not vary by grade level, but differences in performance were found among the various estimation situations related to size and accessibility of the to-be-estimated objects (TBEOs). Concerning the estimation strategies, students used the benchmark strategy, prior knowledge, and a mix of these two strategies, but also less sophisticated strategies such as guessing and eyeballing. A few students did not clearly describe any strategies. The operations of unit iteration and decomposition and re-composition for processing mental measurement were found based on the analysis of strategies and interview responses. To strengthen students' length estimation skills, suggestions for measurement estimation tasks and instruction that highlights students' understanding of size and scale knowledge and their skills to use benchmarks mentally for measurement estimation are discussed.