How do I change font size on a TextButton?

I have written a little script and i want to change the size of the font on it.

I have the script here:

local screenGui = Instance.new("ScreenGui")
screenGui.Parent = script.Parent

local textButton = Instance.new("TextButton")
textButton.Parent = screenGui
textButton.Position = UDim2.new(0, 10, 0, 10)
textButton.Size = UDim2.new(0, 170, 0,25 )
textButton.BackgroundColor3 = BrickColor.White().Color 
textButton.Text = "Spawn"

textButton.MouseButton1Down:connect(function()
    textButton.Text = "Spawn"

end)

If you know how to change the font size it please answer.