I have never seen a terminal solution that allows to have the terminal in fullscreen mode and still have a way to have a user defined writing area which can be resized on the fly.
I have experimented with various distraction free vim plugins but they were all fragile and cumbersome.
If you have a solution, a screenshot would be very interesting.
I haven't maintained a Vim config in a few years now (more of an Emacs man now), but I do remember using Goyo in college. Looking back at it, I think it might scratch your itch as far as Vim plugins go, it even allows you to resize the area on the fly.
Goyo is one of the plugins I experienced as brittle and cumbersome. IIRC it creates the centered text area by creating 9 windows. The outter 8 to center the one in the middle, which is used to edit the text.
In my attempts to use it, that created a bunch of issues.
does it have spellcheck? text-to-speech so you can have it read back to you? A browser window would have those things (on a mac, and I assume there's equivalents in windows)
Funny, Firefox on Mac sure doesn't integrate with the native text to speech, or offer it's own without plugins as far as I can tell. Gee it's almost like all browsers aren't exactly the same
Nothing on my screen except two areas: The writing area and the margin around it.
I want to be able to easily resize the writing area the way I can resize the textarea.
I want to be able to easily change the font size via ctrl+ and ctrl-.