Rear shockers - height?

My 'Dream has aftermarket shockers fitted.  I would like a little more height at the seat.  The rear shocks measure 340mm (just over 13'' in old money) from centre of lower bolt to centre of upper bolt.
Can someone let me know what a new set of shocks would do for the ride height, and what would be the measurement of new ones when fitted to the bike?

Thanks for any advice...