HVAC is science and math just like acoustics.
Lots of suggestions, but no real details on the room. Does it face south with 100 sq.ft. of glass? Is it in the basement, second floor? Are you seating 3 or 13?
Just like acoustics, you can take a best guess based on what has worked for others. Honestly, a lot of what I have seen done works, but at what expense. Slot diffusers are expensive, so did you actually need 8 feet? Do you need 2-slot or 4-slot diffusers?
Oversizing the diffuser reduces throw and can decrease efficiency. It's like washing your car with no nozzle on the hose. The whole car gets wet. Eventually. Best to have the proper size meeting the NC and velocity without making it larger than need be.
Did you really need to run a 10" duct that was difficult and time consuming, or would a 8" have done the job?
Figuring out the cooling load for a single room with only the projector should be a piece of cake (I suspect at least one person posting in this thread can do it for you). Are you doing all the work yourself, or are you going to spec it for a HVAC contractor? [rhetorical question] If you are involving a contractor I would make him figure out the load. Once you have that you can come here and get some great advice on sizing.
But everything depends on what the heat gain is... IIRC you can figure about 1000 btu for the projector and 350 btu per person. Then you need to figure the heat gain for the room based on where it is. Once you have total btu you can determine required cfm and progress to duct size and diffuser size. Two 6" flex ducts? Maybe, but that's pushing the limit for 100CFM, especially in a theater. That's if you need 100CFM and if the length of the run is acceptable. I don't think you'll get 100 CFM through 100 feet of 6" flex.
As for returns (sorry, was your OP and neglected to answer it), I never heard of an issue oversizing it as long as you have a damper to balance it. The return register really can't be too big, AFAIK, other than expense and the size of the hole you're punching through your aquarium.
Not a HVAC professional.. but seen my share.
Edited by Mr.Tim - 6/12/12 at 5:07pm