There are two ways you can do this. 1. a script (which if I hear right, there are some issues which I can't remember) and 2. do it through the main script. First, open Script Editor (F11) and go to "Main" (It's the lastest script).
Place this script:
CODE
Font.default_name = "Font_Name"
Font.default_size = Font_size
You can place it anywhere before this line:
CODE
$scene = Scene_Title.new
It's about how you want to do it, but don't ask me for a script. I don't that kind of script.