The dome part is supposed to be the measurement, with the flange being 1mm bigger, ie 6mm, 4mm. Cost cutting Chinese parts are 0.5mm bigger, ie 5.5mm and 4.5mm.
FZ uses the max to ensure interference fitments because it's safer.
I have to file down rect LEDs to make the 5mm width to fit on a 0.100" PCB because of poor manufacturing tolerances, it's up to you to make the final call.