Hello Nige k,
Perosnally for all my Carp Fishing i used the 12 inch Bank Sticks front and bank (I no longer do Carp fishing) the only time i would use a longer bank stick was if i was river fishing for Carp and that is only to keep as much of the line out of the tow of the Rivers water.
Remember that the 12 inch banksticks are extendable so that size should be plenty to give you a good solid support.
Tight Lines