From Riftui Wiki
The base of all Addons. This is the container for all of your frames. In order to grab information on it and interact with it, you must create a Context.
A Layout's dimensions are 1280 x 1024 and will scale upwards if the user has a higher screen resolution. If the user has a lower screen resolution, values will not go below the default 1280 x 1024. Instead, the UI is scaled down visually but still believes it's 1280 x 1024 
To get the size of the players screen, use:
local screenWidth = UIParent:GetWidth() local screenHeight = UIParent:GetHeight()
If the player enables UIScaling, this methods return already scaled values (eg if the player has his UI scaled to 50%, UIParent:GetWidth() doubles).