You should be able to vertically adjust the relative heights of the revision list and revision diff panes in the revision dashboard, using some sort of grippy resizer.
(This would be a horizontal resizer if you've moved the diff to be next to the list, per bug 854913).
Obsolete with the revamp. It's going to be a lot easier, because you will now open the revision diff right below the current revision list entry. I don't see a need to resize anymore.