I just did this last weekend for about 40 friends,family,neighbors. I used a 5x9 screen hung right on the side of my DJ trailer. 2 T39's to handle the lows and a pair of melded DR250's running in stereo spread 45' apart on stands to handle the rest. Everyone sat up to 50' from the screen with not one complaint with the picture or the sound
I have to say that it is quite the experience to be outside at night and to have Zero Dark Thirty playing at reference level. When the choppers are flying by and you feel every pulse of the chopper blade in your chest,chair,ground beneath your feet. It sure adds a new dimension that you just do not get in a home theater or the big theater for that matter. I have a killer HT and its just not the same as doing it outside. Its how fast the sound dissipates outside that makes it so real.
We ran the popcorn popper non stop and had a hotdog roller handling snack duties along with homemade old fashion ice cream for every one to enjoy while they watched the shows.
Overall everyone that attended was blown away with the experience and can not wait to do it again. I agreed to do this every other weekend if weather and life did not get into the way.
My advice for a successful showing would be as follows.
1. Get yourself a proper screen as large as possible for you given projector. (Look at "Carl's Place" for a great screen at DIY prices)
2. Have everyone that is attending bring there own lawn chairs and or blankets,sleeping bags
3. Have everyone bring a snack for the snack table.
4. DO NOT play TED if children are present. Better yet just don't play it at all.
5. Run a stereo pair of tops and subs as close to reference as possible without pissing off your neighbors.
6. Invite your neighbors so they do not get pissed.
7. Run your popcorn popper off of a separate circuit to avoid tripping a breaker.
8. Safety first when running your cables. People will be in a strange area in the dark and will trip over every cable they can if given the chance.
9. Same goes for your projector. Make sure it cannot be bumped into.
10.Have a good selection of titles to chose from and urge people to bring there movies to view so everyone is happy with what is playing.
11. If kids will be attending play there movie first. They tend to fall asleep by the time its over.
12. If it gets too chilly have propane patio heaters ready to go. We ended up using 4 to cover everyone. (They do a great job of keeping everyone warm without being too loud or bright.)
13.And last but not least, No matter how dark you think it is do not try to fool around with your wife because you will get busted by your buddy's and they will tell you two to get a room.
Have fun and make sure it is a good time for everyone involved. If you come up with anything that I have missed be sure to add it to the list for others to use as a reference. Take pics of your setup and post back with your turnout.