To reduce curricular fragmentation in science education, reform recommendations include using common, unifying themes such as scaling to enhance curricular coherence. This study involved 215 participants from five groups (grades 5, 7, 9, and 12, and doctoral students), who completed written assessments and card sort tasks related to their conceptions of size and scale, and then completed individual interviews. Results triangulated from the data sources revealed the boundaries between and characteristics of scale size ranges that are well distinguished from each other for each group. Results indicate that relative size information was more readily understood than exact size, and significant size landmarks were used to anchor this relational web of scales. The nature of past experiences situated along two dimensions—from visual to kinesthetic in one dimension, and wholistic to sequential in the other—were shown to be key to scale cognition development. Commonalities and differences between the groups are highlighted and discussed. © 2006 Wiley Periodicals, Inc. J Res Sci Teach 43: 282–319, 2006