There are lots of variables when it comes to line length: Sailor size and weight, Seat vs waste, Wind/Sea conditions, style of Windsurfing, Boom Height, Stance, Harness line Brand.
I am 5' 6" and 138 lbs. I sail with a seat harness and am currently using 32" fixed Chinook lines. I "may" drop down to 30". At times they feel slightly long but then other times they feel perfect. For now 32 feels pretty close to ideal but I will experiment more this coming season.
I had a similar question as you when I switched from a Waste to Seat Harness early last season. Please read the thread concerning harness line length I started over at iWindsurf. You may find it helpful.
http://www.iwindsurf.com/forums/viewtop ... a35cf15515In the mean time, perhaps get a set of adjustable lines until you dial in your ideal length. However, I myself checked out the adjustables over at Air Time and North Shore Ski and Board. Their adjustables only went up to 30 inches. So, if you are looking to experiment with lines 32 inches or more you might be out of luck locally. You may need to special order. Good luck!