Which step involves checking both pupils for equal size during the HGN test?

The step that involves checking both pupils for equal size during the Horizontal Gaze Nystagmus (HGN) test is the Resting Nystagmus Check. This procedure is essential because any noticeable difference in pupil size (anisocoria) or the presence of resting nystagmus can indicate potential issues that may affect a person's motor function or indicate intoxication. By assessing the pupils' size before proceeding with other aspects of the HGN test, the officer can gather important information about the subject's neurological status, which could influence the overall assessment of impairment.

Other aspects of the HGN do not specifically focus on pupil size; rather, they measure other indicators like smooth pursuit and nystagmus observable when the eyes move.