Though I have tried to modify \"font.size\" in classes like \"Label_control\" and \"sidebar_control\" in the Package \"Theme-Default\", the font size of the editor does not
Default.sublime-theme file works unless you have installed a theme. If you did, go to your theme's github repo and download the your_theme.sublime-theme file and put it in your 'User' folder.
In that file, find "class": "sidebar_label", add "font.size":16 to that section.