Bra back fasteners are available in various sizes. The SIZE refers to the vertical HEIGHT of the fastener as worn.
Each fastener comes as a set containing two parts: - The hook side, and the eye side. The wider the fastener the more sets of hooks and eyes it will have.
However all sizes may have an adjustment, which is how long or short you can make the fastener if need to make your bra tighter or looser, generally this adjustment is between 1 (no adjustment) and 3 rows of eyes, occasionally we obtain some which have 4 rows, but these are not common.
The picture is normally of the actually product, however, items may change slightly, particulary, the hook side may or may not be prefolded. If the hook side is flat, simply fold it in half over the edge and sew as usual.