The system dimensions are the actual physical size of the units. Extra room needs to be allowed on either side of the filter at the top to connect the inlet and outlet tubes. This needs to be 70-80mm unless John Guest stem elbows are used in which case an extra 10mm needs to be allowed.
Extra space of 30-50mm needs to allowed under the base of the filters for servicing and changing of the filter cartridges.
Changing the filters is quite simple, just a matter of turning off the inlet valve, releasing the pressure by opening the filtered water tap, anscrewing the filter housings, removing the old cartridges, clean the housings, insert new cartridges, screw housing back together and open the inlet valve and flush the cartridges ready for use.