Measurement estimation skills are relevant for our everyday life and should be addressed in the mathematics classroom. Although students' measurement estimation skills were examined in several empirical studies, a structure model to describe these skills is still missing. The current research aimed at analyzing the structure of length estimation skills as one basic measurement estimation skill. Taking into account the different kinds of estimation situations, a test on length estimation skills was developed and administered in two contrasting samples of 3rd- and 4th-grade students from Germany and Taiwan in two studies. Study 1 (N = 248) followed an exploratory approach to analyze the structure of length estimation skills (research question 1). Exploratory factor analysis suggested a 3-dimensional model of length estimation skills, dividing the dimensions (1) estimating lengths of small objects, (2) estimating lengths of objects that are not small but touchable, and (3) estimating lengths of objects that are not small and not touchable. This 3-dimensional structure could be confirmed using confirmatory factor analyses in study 2 (N = 903). In addition, the test on students' length estimation skills proved to be partially measurement invariant across the two groups (research question 2). The cross-national comparison (research question 3) revealed differences between Taiwanese and German students' length estimation skills that correspond to the educational traditions in each country. Taiwanese students reached better results on the dimensions small as well as not small and touchable while German students were ahead in the dimension not small and not touchable.