There is currently no information on the PixelsPerStud or SizingMode properties of SurfaceGuis, as well as the CurrentDistance, DistanceLowerLimit, DistanceStep, and DistanceUpperLimit properties on BillboardGuis. These were all introduced in Version 383 last year.
The SurfaceGuiSizingMode Enum used by the SurfaceGui.SizingMode property also doesn’t have any description.