<p>“Scale, proportion, and quantity” is a crosscutting concept (CCC) identified by the Next Generation Science Standards that underpins disciplinary core ideas across science disciplines. Size and scale cognition is a component of this CCC that research has demonstrated is difficult for students. However, instruments available to assess students’ size and scale cognition are time-intensive, limited in scope, or difficult to replicate. Overcoming these limitations would support the development of tailored instructional supports and facilitate the investigation of student conceptions. This paper describes the development and validation of a novel instrument, the Assessment of Size and Scale Cognition, a computer-based assessment. Task items were developed in alignment with the framework to characterize and scaffold size and scale cognition and guided by already existing instruments. Evidence of validity and reliability was obtained through an iterative process of review among content experts, experts in graphic design and human-computer interaction, and members of the target population. We conducted a pilot test with 518 first-year undergraduate students and used response data to assess psychometric properties. Results suggest this instrument is reliable and can be used to measure students’ size and scale cognition. The iterative refinement of the instrument and stakeholder contributions are discussed.</p>