Sometimes in edits, you can give the clarification for your changes as " added xx characters to body."
Where is the character count visible on screen? I can not find it from the mobile website.
Those edit summaries are automatically generated by the system; others are 'edited tags' and 'edited title'. You do not see a character count on the screen, neither on mobile nor on the desktop version of the site, since it's not important, except for one thing: when you suggest an edit to another user's post, it must be 6 characters at minimum. (If you fail to do so, an error will be shown upon submitting the edit.) Also, you must provide a custom edit summary yourself, so that reviewers will know why you edited the post. Common reasons include "grammar corrections" and "formatting".
If you edit your own post, or have 2,000 or more reputation, you do not need to provide an edit summary, but if the changes are non-obvious it's nice if you do provide one.