These DSLR Bahtinov Focus Masks snap-in to your UV or clear filter that then screws onto your camera lens. So select the size that fits the thread size of your UV filter (not included).
The Bahtinov focus technique works best with longer focal lengths and becomes less useful with shorter focal lengths. The practical useful limit is around 100mm focal length or longer.
(Don’t confuse focal length and camera lens size).