I'm 5'9" with a seat harness. 28 is about perfect for me. I accidently bought a 24cm pair because i couldn't read the size on my old one and it was a nightmare. Way less control, reaction time and I swear I catapulted twice as much.
Great to hear about other seat harness users, it seems that the waist harness is all the rave, but I'm all about comfort and seats works for me. My seat harness boardshort bulge is sure to scare the ladies.