Change font size?

I have a chat gui and when I chat, it adds the textlabel in the player inside a Frame. I want when I click this gui, the font changes to Arial.

a = script.Parent
clicked = a.MouseButton1Down

game.Players.PlayerAdded:connect(function(Player)

a.MouseButton1Down:connect(function(clicked)
	
	if clicked then
		
		game.Players.Player.PlayerGui.ChatGui.Frame:IsA("TextLabel").Font = "Arial"
		
		end
	end)
end)

Thanks for the help!

You need to use enumeration or 'enum' for short it's stupid but you must

a = script.Parent
--We don't need to define mousebutton
game.Players.PlayerAdded:connect(function(Player)
     a.MouseButton1Down:connect(function()
        game.Players.Player.PlayerGui.ChatGui.Frame:IsA("TextLabel").Font = Enum.Font.Arial
    end)
end)