People are creatures of habit, here is mine:
I usually edit dashboard size via Edit... link in the bottom left corner of dashboard frame, where it says Size. When clicked, it opens Page Setup window with Dashboard Size tab active.
File -> Page Setup opens the same window with Layout tab active. I usually use both Layout and Print Scaling, but would have a slight preference for Print Scaling to be active by default in this case.
For dashboards, definitely Dashboard Size. For worksheets, I have the same process and preferences as Dimitri. Though I think from a workflow perspective having a larger dialog and fewer tabs would be better, because when setting up worksheets I'm often in all 3 tabs.