I set this: PCB Editor / File / Board Setup / Design Rules / constraints / Silk / Minimum text height to "0.6453" in a random project, saved it, and then did a grep on the project directory for that number.
Grep shows there is now a "min_text_height": 0.6453, in the [Project].kicad_pro file on line 154.
Just like all other KiCad files, the project file is in a human readable ascii format. Therefore you can edit them in a text editor, use a script that does bulk modifications in a lot of files, etc.
Some Programming IDE's have built in functions to search for and replace text in a whole directory tree. (Monodevelop had that function, R.I.P).
But please be a bit careful when you are doing this. Editing by humans or external scripts is not a main goal of KiCad, and KiCad does not do much syntax checking, so a forgotten bracket or comma confuses KiCad and is hard to fix afterward.