42mm x 11mm festoon, i believe its also known as a 264 bulb size.
If you thinking of getting a LED one be wary, i got one for my car and it has 3 LEDs, looks good, but the light it gives off is poor if you actually want to find something in the car in the dark!!
I usually put the original one in during the dark months!
EDIT: I should also point out that i got a RED coloured one, so maybe a white or blue one might be better,
or look for one with more than 3 LEDS, plenty of room in the light so if it was a bit wider than the original [11mm] it would still fit
id say.