I just replaced one of my license plate lights. I believe it's a 36mm Festoon bulb. But I'd have to measure it.

I would NOT recommend putting a halogen in this spot as the bulbs are actually mounted in the plastic lens 'ears' and then two screws hold it and the lens in the socket.

I would do bright white (2 or more) LED Festoon bulbs from SuperLumination

I'll probably change all my regular bulbs to LED lights soon. I have purple (UV) lights in my trunk area and dome lights.