For what is worth, on desktop Discourse already remembers the last editor height manually set by the user. In principle, every time you reply to a comment, the height of the editor should be the same than the previous time.
However, if the user sets the editor to full-screen, it seems that Discourse won’t remember it next time. @awesomerobot Maybe a logical (no idea how simple) solution would be to add fullscreen as an option to be remembered by the editor?