how do I change a textlabel size using a script ?

this should be changed with Udim2 like this and change it in PlayerGui to let the player see the change like this

game.Players.LocalPlayer:WaitForChild("PlayerGui"):WaitForChild("ScreenGui").TextLabel.Size = UDim2.new(0,100,0,0)

the second and last values are offset for x and y

If your looking to use scale first you wanna find how big you want it to end off as. I'll use 0.3,0,0.13,0 for this

What you need to do is simple: GuiLocal.Size = UDim2.new(0.3,0,0.13,0)

for what your doing (In a local script anywhere):

local Plr = game.Players.LocalPlayer
local Gui = Plr.PlayerGui.ScreenGui.TextLabel

Gui.Size = UDim2.new(0.3,0,0.13,0)

Hope this helped