Install gnome-tweaks, change “Fonts > Preferred Fonts > Interface Text”. Evolution follows the GNOME desktop settings for fonts as there isn’t much sense in having different fonts in every application.
I am using Cinnamon and am happy with the font sizes for everything, except Evolution where everything is noticeably smaller. It therefore doesn’t make sense for me to mess around with the fonts of my entire DE (if such tool is even possible for other DE’s).
Evolution does not provide per-application settings as it would make no sense to have different font sizes in different application. As Evolution is a GNOME application I already explained what you could do. Have you tried?
To add to what Andre said, Evolution is a gtk3 application, thus it uses whatever the gtk3 has set. Where it’s set depends on the desktop environment (for example KDE has a special section for gtk apps). The gnome-tweak tool may do the job for all the apps. Alternatively, you can add/edit ~/.config/gtk-3.0/gtk.css file and add to it: