Max is 4" on stock arm because as you extend arm you widen axle length . I have done many 3" ones and stock shock still works but not a stiff. 4" or longer shock must be taller for stock height. Now you could get another swing arm from another bike like a GS750 and with a little work you don't change the axle length. I have used GS GPZ and CB arms on XS650 for a little stretch and all worked or you can buy a aftermarket square tube arm that is made for your bike too but they are costly.