Photoshop color mode for print and web document?

Both.

You can save copies in either color mode. At the very least, you will need to verify that the CMYK version is acceptable, since it has less tolerance for bright saturated colors that cannot be printed with basic inks.

The web version can be in RGB mode without a color profile (or set to sRGB). The print version should use whatever color profile that is appropriate for your printer. If you are sending them off to a service, ask them where you can download a suitable color profile.

Design/draw/paint it in RGB with your color profile set to sRGB. Set your proof setup to Working CMYK (hint: View > Proof Setup) - toggle in and out of the proof colors preview using View > Proof Colors - or work using the gamut warning.

Then once you have finished and saved your PSD (in RGB mode), only convert to CMYK TIFF for printing processes such as lithography or screen printing which require separations. Be careful you do not overwrite your original RGB PSD.

If it's just regular inkjet or digital printing - you do not need CMYK images. In fact some inkjets can't handle CMYK images at all, and many digital printing services prefer RGB images. Best to check with your printer to find out what they need. Don't just take it for granted.

CMYK images will not display properly in a browser. RGB is required for images on the web. If it's intended for the web you will also need to make sure you are using the sRGB color profile if you want color fidelity.

Yup which you can design the required measurement poster on photoshop for instance 3 ft * 2 feet means your design have got to be set as 36 inches * 24 inches and set a resolution of minimum 50 pixels

rgb