After I maximize the viewport window, I can really get these 2 sizes through ga_viewport_size_get(). However, I want to get these two values before maximizing in order to call ga_viewport_size_set() to maximize viewport window.
I suggest posting one viewport only and then calling the function:
uil_viewport_tiling.tile( )
this will maximise the current viewport to the size available (which is probably what you want) then you can query its size with ga_viewport_size_get if you really want to know the width and height available.