How can I change the font size in this script?

I have created a simple script that shows group ranks over our head, but what I'm having trouble with is setting the font size. I'm using the function Enum.FontSize.Size, but it won't change the size of my billboard gui. I would also like to know how I could change it to something smaller than 8.

game.Players.PlayerAdded:connect(function(plr)
plr.CharacterAdded:connect(function() -- So a new character is in use.
    local rank = plr:GetRankInGroup(2696373)
    local role = plr:GetRoleInGroup(2696373)
    local size = Enum.FontSize.Size8

    if rank ~= 0 then
        local rankd = game.ServerStorage.Rank:Clone()
        repeat wait() until plr.Character
        rankd.Parent = plr.Character.Head
        rankd.TextLabel.Text = role
        rankd.TextLabel.TextColor3 = Color
        rankd.TextLabel.FontSize = size
    end
end)
end)