The big room sounds awesome - but it really depends on how much time, effort, and money you want to invest into a home theater for the room you use.
Either room is of deecent size, but the 15x30 foot room (5mx10m) really sounds like it would be ideal.
As Clint says - the going rate is at least 1.5x screen width - though many would seriously argue the 2.0x width with the current generation of 720p LCD projectors. But, few will argue that if you have the budget, a nice DLP is the way to go... except for some LCoS lovers.
There is some good information here, and some excellent information on
www.projectorcentral.com that you can read about.
Without you having a specific budget set up yet (which is hard to do, I know) it is hard to say what any of your options are. But, in the big room, if you went with a 10 foot wide screen, you would put your first row of seats about 15 feet away, then a second and third row would be possible. You might want to consider a rear projection room with a long throw projector - this remove projector noise from the room and makes it feel more 'theater' like.
Other considerations in your budget should be:
1. Audio
2. Video Sources
3. Control of everything (don't have 5 remotes out in a nice theater!)
4. Room aesthetics.
5. Furniture
6. LIGHTING - One of the most often overlooked aspects of a nice theater is giving it good lighting with lots of loads and multiple zones.
Good luck!
