There is apparently no way to change the font size of the tool bar at top. If I change the font size for the menus in all applications in the Windows settings, it won’t change the font size of the eM Client tool bar. It seems to be hard coded in eM Client. I opened a support ticket on this a while ago, but it did not help.
You can change the font, and its size, of the folder and item lists here: Menu > Settings > Appearance > Lists > Fonts.