It may be of use to others how I figured out the size I needed: Wiha Tool and probably others measure Torx bits across opposing points. My calipers are too thick to measure this accurately, and the bolt's in a location that's hard to reach, and I'd have needed more hands and younger eyes than I own to measure this directly. I cut a piece of sheet metal to the rough dimension and filed it until it fit into the opening across two points of the "star," then measured it.