  Referents are often used to enhance scale perception in immersive
visualizations. Common referent designs include the considerations of referent
layout (side-by-side vs. in-situ) and referent size (small vs. medium vs.
large). This paper introduces a controlled user study to assess how different
referent designs affect the efficiency and accuracy of scale perception across
different data scales, on the performance of the size-matching task in the
virtual environment. Our results reveal that in-situ layouts significantly
enhance accuracy and confidence across various data scales, particularly with
large referents. Linear regression analyses further confirm that in-situ
layouts exhibit greater resilience to changes in data scale. For tasks
requiring efficiency, medium-sized referents emerge as the preferred choice.
Based on these findings, we offer design guidelines for selecting referent
layouts and sizes in immersive visualizations.
