I'm running a EO snuffer (same as the I-20's) that is 12' 2'' in length (I believe). I don't know the square footage of my spinnaker, but it's a smaller one.
I have 3 spinnakers that came with my boat when I bought it. The two larger ones use rear beam blocks and I've never liked the performance. I've gone back to the smaller spinnaker (with blocks off of the side stays) and found it to work much better. I could be doing something completely wrong, of course, so I might not be the best person to get advice from.
I'll try to get some measurements on the spinnaker the next time I take them out of the bag.
My main spinnaker problem is that the one I like to use doesn't go completely to the spinaker halyard block at the top, so I don't think it's a stock "Mystere" blessed spinnaker. I have to leave about 12" of slack at the tack in order to get the spinnaker to travel to the halyard block with proper luff tension (a handful of "give"). Not sure if that is good or bad. It seems to be working well . .. so .. . I'll just go with it.
If you need any more info, let me know.
Boatless and happy.