Standard flush mount boxes fit 55mm holes and there are different types for drywalls (with hooks) and to be plastered in and different depths for more connectors in the back.
55 mm would be a bit small... the boxes themselves are 60 mm, drywall boxes are even larger at 65 mm (have a look at the Kaiser catalogue!). An 68 mm holesaw is perfect for drywall boxes, for masonry boxes you'd typically use an 83 mm diamond hole saw and plaster.
