According to Mesbursmith in a previous thread, you are right Mike, the number for requiring a second exit is normally 50. Of course local codes and your experience may vary.
http://www.bigscreenbiz.com/ubb/Forum47/HTML/000053.html
If you can only provide the one exit, then you should easily be able to fit 49 people in a 52x20 space. The proportions are pretty good with length equal to about 2.5 times the width. I currently have architect plans in hand for an auditorium that is more like 28x13 (plus 3 feet behind the screen) and seats 31 plus 2 handicap spaces. Oh yeah, you are still required to provide ADA accessible seating, all the way down to rooms with a seating capacity of 4 people. Can you get a wheelchair into this space, in a manner consistent with building code?
I'm not sure how the 14 foot ceiling figures in, but I'm pretty sure that it shouldn't be too much of a problem with less than 50 seats. It seems to me Avalon built his 4-plex with similar ceiling heights and rooms that seated 49.
I'm just wondering how you are going to manage how people enter and exit. Are you going to station someone at that door just to watch it? It sounds like the space isn't connected to the rest of the building if people have to enter and exit only from the outside. That means people would be free to come and go as they please if there isn't someone watching.
Also, how are people going to have access to restrooms, concession stand, water fountain, etc? Will they be required to go around the outside of the building? I'm not sure if that satisfies building code, and it certainly doesn't sound convenient. Maybe you could build a corridor around the outside of the building to connect the space and create access control and make services more accessible , but then you will need to be careful about the distance people would have to travel to exit. And again, will the required services be ADA accessible?
Lastly, are you going to have to create a new projection booth to service this space? Is it going to be isolated from the rest of the complex as well? That makes it harder for your projectionist to handle it along with the other booth(s).