You can add Logo files in the Basic Setup. You can also upload their Logo Reversed that can be used if you have some pages with a dark menu or transparent menu for better readability.
To change the logo size in the menu you will need to edit each Layout you use.
Layout = header (menu) + <page-content> + footer
You can select it via /wp-admin/ panel this way:
Edit Menu and change logo size
e.g. to 90
to change tablet/mobile size please change it this way
Click Update to save changes
If your website uses more than one layout and you wish to migrate the menu (e.g. some homepages and blog pages use by default transparent menu layout) please edit other layouts and change it the same way again:
