UDim2

The UDim2 data type represents a two-dimensional value where each dimension is composed of a relative scale and an absolute offset in pixels. It is a combination of two UDim data types representing the X and Y dimensions. The most common usages for UDim2 are setting the Size and Position of GuiObjects.

local guiObject = script.Parent
guiObject.Size = UDim2.new(0, 300, 1, 0)  -- 300 pixels wide; full height of parent
guiObject.Position = UDim2.new(0, 50, 0, 0)  -- 50 pixels from the left