I have a couple of different themes, but I'm having trouble getting them to look the way I want at 100%.

The mobile navigation, when set as a sidebar, always disappears over 90%. And the sidebar on the right, with widgets, gets pushed to the bottom of the page, too.

SoI would like to set the ideal viewing size for new arrivals (or just for myself in fact!) to see it optimally. This viewing size varies across themes, ranging from 75% to 90%.

Is it possible to do this through the ACP or CCS?

Im honestly not sure what yo umean by the ideal viewing size, to be honest. This is really subjective. It depends on how you want things to look