On both the legacy edit page and the visual editor edit dialog, I think we should be encouraging lots of prose in the edit summary field. It seems there is no consensus about this yet, it's implemented differently in many places. Whatever decision is taken, we should be consistent across editor interfaces, upload wizards, etc.
This is the editor's big chance to explain "why" they made an edit. Sometimes a short message is appropriate, and other times a longer message, relevant URLs, and so on.
Current behavior is badly broken for both editing interfaces, if there's too much text it will be scrolled horizontally or vertically in a tiny box. At the very least we should be auto-sizing the field. Even better, start it off as three lines tall, and provide the resizing handle in the corner. Do something reasonable about handling the <return> key.