I would like to present a full-screen text editor. Are there any standard rules for calculating the side and top and bottom margins based on the screen size and possibly font size?
I'm particularly interested in mobile and tablet devices, in all their possible orientations.
It's hard to say if you refer to user experience only and depends very much on the purpose of your full screen text editor.
Regarding programming, there is (used to be?) a standard of 80 columns - you can read about it here http://www.emacswiki.org/emacs/EightyColumnRule, it says:
The sacred 80 column rule states that “Thou shalt not cross 80 columns in thy file.” This initially was to make printing easier. Nowadays, the newer editors and users of editors have forgotten this and make it really hard to read code.
But this "best practice" derives from the limitations of the past, as you can see (you have line wrapping after all).
On the other hand, there are text editors that are more aimed at the limitations of user perception. Wider text is harder to read, and thus also harder to edit, to some point of course. Bigger margins make it easier, but it's hard to say what is the perfect width, as users are different and have different preferences about it. So best practice, methinks, would be to refer to this preference and put it in the hands of users.
There are some best practice examples, one of which may be Byword, a distractionless editor (available for both OSX and iOS) where user can choose between wide, standard and narrow layouts: http://bywordapp.com/
