In my experience, LED lights are annoyingly bright to look at, so I would go as small/dim as you can find. In fact you may want to consider just drilling a tiny pinhole in the gauge and mount the actual LED bulb behind it. You can experiment with how bright it will be by mocking it up on a piece of sheet metal with various sized pinholes.
I have seen this done on an AD dash and the holes were very subtle and the lights were plenty bright.


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission