There's absolutely no documentation for it, that's the problem. If you google the function name you can actually find it used in a .cpp on (IIRC) Landoleet, Justin's dev server.
I've been able to do this much with it in Lua:
Code:
local _, _, w, h = reaper.my_getViewport(0, 0, 0, 0, 0, 0, 0, 0, 1)
reaper.ShowConsoleMsg("Screen width = "..tostring(w).."\nScreen height = "..tostring(h))
- The 30px missing from Height are my taskbar, I think.
- It has issues with multiple monitors.