Do you really mean 10 x 20 feet? What sort of illustration or graphic is it? What would a typical viewing distance be? What is your computer spec? A file that size at 300PPI is going to be 72,000 x 36,000 pixels. It created a 12Gb scratch file before I'd given it any content, so is going to put a lot of load on system resources.
The colour space, as with CMYK, doesn't have any impact on resolution, other than if it is CMYK I'm guessing that you are not the person who will be printing it, so you should contact the printer and ask what they need. I tend to let my printer do the CMYK conversion, because he is better at it than I am. He always gives me a test print to check before committing to the full size print.
Take a look at this , which may help you.
Trevor's advice is spot on - talk to your printer. If you are working in CMYK you also need the specific CMYK space from your printer (there is no generic CMYK)
Thanks for sending information, if i set 50 resolution in size 240x120 inches, then what type of result will be show on out put in flex printing.