Quite a while back SourceTree's commit dialogue was replaced with the commit panel on the bottom of the window in the File Status tab. It uses a lot of screen space when trying to manage many changes or viewing a long diff in the right-hand side panel. Especially when using a small resolution such as my laptop's 1366x768.

I suggest adding an option to collapse/hide/resize it when you're not writing a commit message. Clicking the collapsed panel or the commit button would restore it.