Make it in inches, not pixels. Make it the full size of your CP and at least 300 dpi.
D
Inches are good. You can type any dimension into any dialog in PS, and it will translate.
The resolution you need depends on the printer you're sending it to. On my printer, anything over 100 ppi is pretty much a waste. It can render vibrant vectors up to 300 ppi, but not pictures, and nothing in subtle shades.
Making a CPO in photoshop will be very tedious. You can use the vector tools to layout the controls, and it will be great for that, but when you get to putting artwork on it, it will get old fast if you try anything complex.
Bob