This userstyle displays the comments in a large font for the All Comments Frame opened in a new window/tab.

I like to read the comments in a new window so I can scroll the comments in one window and watch the video in another window.

If you right-click the All Comments hyperlink
and choose "this Frame" > open in a new window (or tab)

Attention - After the window is opened resize it a little so the comments width gets "fixed".

Instead of this style you can install my http://freestyler.ws/style/47162/youtube-bigger-font-for-description-and-comments

See also my http://freestyler.ws/style/19765/youtube-blue-gray-background