It looks like that can't be done through the GUI. For the moment you can edit settings.reader.lua.
Make a backup first if you don't want to risk losing your settings if you make a mistake.
It'll be the line that says something like:
Code:
["copt_font_size"] = 22,
You can change it to anything, so also something even more finely tuned like 24.1. Not everything will result in a difference in pixels of course, but because the GUI was originally created for Kindles from 2012ish finer per-pixel details might potentially be lost on higher DPI devices.