The hose diameter is actually not specific, and its only the banjo end that can be 5 or 6.
The fittings for the banjo and banjo itself are thinner, but you can take a 5mm hose(inc 5mm fittings) and fit that.
TBH I dont know if it matters greatly in performance, but ive come across lots of early Hope with mix and match 5 or 6mm hoses/fittings