SourceTree already has an optional column guide for commit messages, but many git users follow Tim Pope's recommended commit message convention (http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html) which suggests limiting the first line to 50 characters.

The vim-git package supports this by highlighting trailing characters in red. I usually try to write commit messages on the command line so I can see if my first line is too long, but it would be nice if SourceTree supported it directly.