I have to make artwork for a banner that is 123'' X 194''. I am not sure what size to make the page I am creating the artwork on in Photoshop. It obviously doesn't let me make it that big. Do I need to make it into CM or MM? Thanks in advance
I can. What version of Photoshop are you using?
Note that the size mentioned at 300 DPI is 6GB in size, so that's pretty large. Also, I get a notice (using CS4) that it may not be compatible with older versions of PS. That might be your issue.
Personally, I would "create" the graphic in Illustrator if you have it (or at least the smaller parts of it).
To answer the actual question: You need to (should) make it the size of what you need it, so 123in x 194in @ 300dpi would be your best bet for maximum quality.