How do i fix a gui that is to big on mobile?

GuiObjects have two Size / Position Properties Associated with the X-Axis and Y-Axis named "Scale" and "Offset".

Offset refers to pixel size which are best compared on a Computer / PC screen

Scale refers to the object's size relative to its parent.

If you want your Gui to look nearly the same on all devices, you have to make sure the Size proportions are Scale only. For Example:

script.Parent.Position = UDim2.new(1, 0, 1, 0)

In the above example, the scale property is 1 * 100% (100%) of the size of the GuiObject's parent.

Another thing to check is to make sure that if your Gui involves any TextButtons / TextLabels / TextBoxes that the TextScaled Property is set to true.

In the Properties Window, the above set-up would look like:

{1, 0}, {1, 0}