If you couldn't find the menus in KOReader, I can see why you would have problems. The thread that contained the installation instructions also has a number of posts that might have made it easier, tips and hints as well as links to KOReader's Github Wiki pages. Poking the top of the screen will bring up the menus. When you bring up the menus while reading, you will have menus at both the top and bottom of the screen. The font size menu is among the bottom menus. The menu to select from the fonts, including any you add to the fonts folder, is among those in the top menus. The menus are grouped under tabs/icons and are somewhat context sensitive, in that what is presented depends on which reading program is used for that type of book/document and whether you are in the File Manager.
Koreader is totally customizable. It is coded in a human readable scripting language and can be easily modified just using
Notepad++ , a free editor.
If you prefer the database oriented book management, with "collections", over file management, then KOReader and Coolreader might not be for you.
Luck;
Ken