What is the resolution of the Apple Thunderbolt Display's built-in camera?
4:3 or 16:10 or 16:9 is aspect ratio.  As for maximum resolution, open Quicktime to do a video recording thru the camera and set the quality to maximum.  You don't have to hit record, just do ⌘+i for the movie inspector and you will see the recording resolution there.  The camera will record at the highest possible resolution it can.