I found the menu for Dialogic 2 (Alpha):
Go to the "Dialogic" tab on the top, then select the "Styles"-tab and then depending on the preset (it works for the default "Visual Novel" style) you have selected there will be a (font) "Size" option.
Note: It didn't work right away for me, but after starting the scene a few times it worked (maybe just restart Godot or so)