Sorry to be late to the party...
I used to use swords around 38" long, exclusively. The style I fought with at the time, around which I made my intro sword & board videos on YouTube (look it up, channel "dagorhirfighting") was quite a static style, in which I would basically stay in one place, throw trick shots, and snipe shins and shoulders. These swords weighed around 20-22 ounces but were super fast, mostly because all the weight was in the grip and pommel and the blades weighed maybe 6 ounces, tops.
Since fall 2008, however, I have been using 32" swords in the 16-18 ounce range. Simple: I am a pretty fast guy, I (potentially) have good footwork, I'm not that tall (5'8") and I like to stab. I realized that I was relying too much on my weapons tech (long, very well-balanced swords) to do the work for me, and not developing my own skills in any meaningful way. That was why I got beaten pretty badly once people figured out my trick shots.
My current opinion is that a 32-36 inch sword is best for most people, for these reasons:
• A sword shorter than this will leave you with too little range.
• A sword longer than this will extend your range farther than you need it (usually) and will make it hard for you to do anything at close range.
• No matter which construction method you use, it is hard to make a 32" sword that is too heavy. Even if it's not balanced well, it won't slow you down too much. (this is mostly for newer, less experienced weapon makers)
• A 32" sword is a lot easier to transport than a 40" sword. Really, this could be important.
Additionally, if you vary your grip effectively, you can extend your range on the fly.