People often assume anything that's printed must be built at 300 dpi/ppi. Fact is, 300 ppi is an arbitrary value that's easy to remember. Beyond that, it's fairly useless. What's important here is to know where and how your job will be printed. As Bob said, 100 dpi or less is often more than sufficient for large jobs like banners, billboards, etc. My advice is to first speak to someone who will be handling the printing. Let them tell you the correct resolution and color space to work with. Don't assume all print jobs must be delivered as CMYK. Doing so will save you a lot of time, aggravation, and storage space.
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