well, that is a problem inherent in any open competition event. When people from different systems compete, it is highly unlikely that any of the judges will have the background to appropriately judge all of the competitors. If you as judge do not have experience with the system and form the competitor is competing with, how do you judge it? You don't even know what you are looking at. You don't know if it was done well or not.
Sorry, I don't have an answer for you, I don't think there really IS an answer to this.
Another question: does a more "complex" form automatically win out over a more simple form? Even if the simple form is better executed?
When ever I am up against this problem, I will ask that the kata be preformed again. The next time around I look for the foundation of the kata, that is inherent in all forms. That being balance, smooth transition, explanation of Bunkai, and spirit. I feel that length is not a good gauge, but that content is.