And the menus in the Comcast box vary with their box and software so I can't be exact here.

READ THIS LINK -> http://www.cnet.com/how-to/tips-on-setting-up-the-xfinity-x1-cable-box/
However CNET notes this:
" In Video Display, you may need to tweak two settings to adjust the video output so that it matches the aspect ratio and resolution of your TV. If you have an HDTV -- and odds are you do if you have the X1 box and are reading this blog post -- then make sure Zoom is set to None. Any magnification of the screen image results in a degradation of video quality. For the other setting here, Video Output Resolution, make sure it is set to the resolution that matches that of your HDTV. "