Hopefully your belt is marked, the first numbers are the length and then the pitch (e.g. 3M, 5M, 8M etc) then its the width, manufacturers usually never mark them with the width of the belt, you would need to determine this, all in millimetre dimensions.  If the timing belt isn't marked adequately, there are times when the timing pulleys are marked adequately to determine the pitch and profile of the timing belt. The pitch of the timing belt is the distance from the centre of one tooth to the centre of the next tooth.  If you can determine the exact pitch, you can narrow down your choices rather quickly by measuring the width of the timing belt and the overall length of the timing belt. Alternatively if you cannot measure the length successfully, you could count the teeth on the timing belt and multiply by the pitch to get the length of the timing belt.