I wish you well with this swap but, IMHO, I am afraid this swap will be a disaster if conducted as proposed. It seems that it is hard enough to get people to tie a mere 12 flies within the time limit set (no matter how long or short). Many procrastinate to the last minute and then are shocked that they have "life emergencies" arise that will prevent them from meeting the deadline and it only takes one or two to mess things up for everyone. This has been my experience here every time I enter a swap. I am still awaiting the flies from a swap I entered back in early February. Extending the time does nothing to improve meeting the deadline. People will only procrastinate longer. Adding more flies will surely insure failure. It may, in some cases, make posting them more expensive and therefore add another layer difficulty and create another obstacle for success.
Stick to the tried and true formula here is my advice. 12 people...12 flies...4-6 weeks turnaround.