The comment box on the rule flow only has space for three lines.

It has a scoll bar but such a limited amount of space make it very hard to enter and review typical comments (which can often run to 20-30 lines for real world rule models. An in place pop-up text editor might be more convenient than having to locate the comment box in the properties tab. In fact being able to edit the comment right in the displayed window would be best of all. See associated suggestion about making comments permanently attached to the diagram rather than hovering (because the hover is only visible for a few seconds)