After reading it twice, I think I understood. The size guidelines are included above. For example:
I think in general this guide is quite outdated. You can no longer add a logo or configure the look and feel in the wizard. The themes admin page looks totally different now, and color schemes were renamed to color palettes.
The dark/light mode toggle theme component was moved to core. And branding has a new config page at /admin/config/logo.
I just wonder why the size mentioned on the config page is different from the description when I look at the site setting.
600×80 is different from min. 360×120.