The maximum approved brake disc diameters are those that were originally installed on the models of the relevant series and the relevant model year. These are valid for both front and rear, even if the fork manufacturer releases larger brake discs for its fork models.
Here are two examples:
The SE 7000 ’08 and SE 9000 ’08 have the largest brake discs in this series: 180 mm front and rear. This means it is permissible to use 180 mm or 185 mm brake discs on all SE bikes from the model year ’08, as long as the fork manufacturer approves this brake disc size on the front wheel! If a fork manufacturer only approves 160 mm brake discs on the front wheel, you may only use 160 mm brake discs.
The ERT models from ’07 have 200 mm / 203 mm discs on the front and 180 / 185 mm on the rear. This means it is permissible to use these maximum disc sizes on all ERT bikes from the model year ’07. If a replacement fork is used, you must enquire as to whether the relevant manufacturer approves this brake disc size for the fork. If the manufacturer only approves discs up to 180 mm or 185 mm, then in this case discs of 200 mm / 203 mm may not be used, even if the frame design allows such use.
The reason we limit the brake disc size for forks even if the fork manufacturer approves larger discs is that braking and the associated loading affects not only the fork, but also the frame, and more precisely, the head tube.