19 mm refers to the diameter of the spindle. For BMX, they come in 19 and 22 mm versions. The BB bearings are specific to the axle size, but they all fit the same frames. All the modern BMX bikes round here seem to use the press-fit "mid" BB bearings. Threaded bottom brackets are referred to as "euro" BBs in BMX, so that's probably what you're after if you're fitting them to an MTB.