How do i make a GUI so when you roll your mouse on it it will change size?

I have tried

if button.MouseButton1Roll then
SizeVar = 15 --I don't know what variable to do so i added this one
ChangeSize (sizevar)
function ChangeSize --this was for if it didn't work
if button.MouseButton1Roll then
SizeChange = 15
ChangeSize (sizechange)
SizeChange.:connect
ChangeSize :Start
wait(10)
ChangeSize :Done
end