How to change font size and font for all profiles in terminal?
Very easy with an AppleScript code (here an example where all profiles are set to Menlo Regular 18 pt, note that "Menlo Regular" is named "Menlo-Regular" in font name):

tell application "Terminal"
    set ProfilesNames to name of every settings set
    repeat with ProfileName in ProfilesNames
        set font name of settings set ProfileName to "Menlo-Regular"
        set font size of settings set ProfileName to 18
    end repeat
end tell