The downside of the high pixel count of the M1 MacBook Pro 16″ is the size of the font in the Apple Mail sidebar, the Safari sidebar and Safari Favourites Bar.
To increase the font size in the main body of Mail is easy enough, but how to increase the font size of the sidebar? The answer is System Preferences > General > Sidebar icon size: Large.
That does not increase the font size of the sidebar or the favourites bar in Safari, however.
I will add to this post as and when someone tells me how to do it.