I came up with the following measurements
Cup that goes into the steering wheel on the back of the emblem: Outside diameter - slightly larger than 2 7/8 inches (74mm), Inside diameter, slightly under 2 3/8 inches (67.5 mm)
The "button" part is 3 1/2 inches across (89 mm), the area of the marquee is 2 7/8 inches (74mm).
The marquee is removable, so if you have a Midget horn button and can find an Austin Healey marquee, you can swap them out
The one eBay looks virtually identical to mine