Gui Help? [Script Included]

How do you changes the size of a gui vertically but doesn't change the bottom size of the gui, but it does it smoothly and randomly up to a certain size? I get that you would most likely use UDim2 but it never works for me, help?

script.Parent.Frame.Size = UDim2.new.math.random(0,100,0,550)

I seem pretty dumb right now if I think thats all I need, but it doesn't do anything. Right now the size is set to (0,100,0,0) and its positioned at (0, 20,0, 560), It doesnt do anything, and if it did work, it probably would change the size of the bottom too. Help?

-- edit to fit your response

while true do
	local random_size_y = math.random(10, 200)
	script.Parent.Frame:TweenSizeAndPosition(UDim2.new(0,100,0,-random_size_y), UDim2.new(.5,-50,.5,0),"Out", "Linear", 1, true)
	wait(1)
end