Gui Text Size?

-- This is just an example
local TextLabel = Instance.new("TextLabel")
TextLabel.FontSize = "Size18"

You can't change the size to "Size15" for example. ROBLOX made sizes that you need to choose from which are the following: - Size8 - Size9 - Size10 - Size11 - Size12 - Size14 - Size18 - Size24 - Size36 - Size48

Hope that helped :)

Scriptree, that can also be used for a countdown. Such as:

script.Parent.Text = "10" 
script.Parent.FontSize = "Size48" --Size
wait(0.1) --How long you have to wait until the next command is gonna run
script.Parent.FontSize = "Size36"
wait(1)
script.Parent.Text = "9" --And so on..

Its good for any countdowns you can use in you're game! Hope this helped out. If you want the script to make a countdown, heres what you should type in:

Instance.new("TextLabel", game.StarterGui.ScreenGui) --Creates the object and where it the script puts the object. ex: Instance.new("Part",workspace)
print('textlabel created') --Check if the TextLabel WAS Created.
tl = game.StarterGui.ScreenGui.TextLabel --Detecting the text.
tl.Transparency = 1 --Make the background transparency 1!
tl.Text = "10" --And so on, the text countdown.
wait(1)
tl.Text = "9"
wait(1)
tl.Text = "8" --And so on..

If you have any problems with the first or second script, just ask me! I would be happy to see if something is wrong. I hope this helped you a lot more. Thanks! :)