If I read your measured drawing correctly, the diameter across the threads is 2.86mm. If that's right, then it is most likely an M3 screw.
These are not hard to find. If you don't have an assortment of metric screws and nuts at home, you can take one to your local hardware store, like an Ace - they have metric screws that small and they have the nuts to match. You can play with the sizes in the box at the store until you confirm what nut smoothly threads on to your screw. If you don't have any of the screws, just the car, then take the car to the store and test with some M3 or M2.5 screws until you make sure of the thread. There are different thread pitches for these, so be aware. Hardware stores usually list them as "fine", "standard" and "coarse" or something like that. You could have an M3 "fine" or an M3 "standard", for example. You will have to just test your screw on nuts in the store. to figure out what you have.
