Sheepie, I hope you are going to give as much consideration to the style and quality of paddles as you do to the length. That's not to say that I believe length is a minor factor, but it is somewhat dependant on personal choice.
You might be able to get an educated estimate from the folks at Werner on their website.
I might be out of line here for assuming that you are relatively new to paddling, but I would strongly suggest that you steer clear of the plethora of cheap paddles that are on the market these days. I can't believe the junk that our local sporting goods store has chosen to display. They used to have some pretty good paddles, but it looks like cheap is the new wave. Go to a real kayak shop.