In the world of marching bands, for each part you have to fit a whole piece of music in a single landscape page. So you have to tweek the staff size a lot in order to get a proper readable part.
The problem in Dorico is that the option to change the staff size is in Setup Mode, so you can’t check it properly in the Engraving mode. You have to open Setup Mode, look for the option and then, open the engraving mode without closing the menu.
The worst part is that when you change a setting, you have to push “apply”.
I think this option (and the other engraving related ones that you can find in the Setup Mode) will be much more useful in Engraving mode, and easily accessible (in the lower zone or anything like that).