On my computer, First[WindowSize/.Options[EvaluationNotebook[]]] gives me 1432.5 if the notebook has maximum width for my screen.
My screen resolution is 1920*1080 however. So the result can't be in pixels.
What's the unit returned by WindowSize?
Another strange observation: I can make the window narrower, e.g.
SetOptions[EvaluationNotebook[], WindowSize -> {716, 760.5}]. If I look at WindowsSize programmatically I get
WindowSize /. Options[EvaluationNotebook[]]
{716.25,760.5}
What causes the extra 0.25 whatever-units width?