The focal length of the lens is more or less irrelevant to the size of lens cap or filter thread it has. Other than the fact longer lenses tend to use bigger lens caps due to the front element being bigger, or smaller lenses with wide apertures use bigger ones also.
The filter thread will be either written on the front of the lens denoted by a circle with a line through it, or on the back of the existing lens cap if it has one. Failing that, google it.