From Tesla Model 3 owner's manual:
Note: Tesla recommends using a flash drive with as much available storage as possible; saving the most recent ten-minutes of video requires approximately 1 GB, and the hour-long video footage loop requires approximately 8 GB of free space. If your flash drive does not have sufficient storage, an "X" displays on the dashcam icon and dashcam may be unable to save video files.
...
The dashcam temporarily stores approximately one hour of the most recent video footage on the flash drive before new video begins overwriting old video.
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So, for the dashcam feature (not consider sentry mode / saved videos) I think the *minimum* you need is "approximately" 8GB, although, of course, larger sizes are recommended.