There is no "always" as there is no "normal size". If you show a window, it has the size of its sizeHint(). If the user resizes it, it does not have that size anymore but rather the size set by the user. So which of the two do you consider "normal"?
Normal - the size a window has when it is not maximized, minimized or fullscreen. Once the user resized it, the normal size is the size set by the user.

You can reimplement resizeEvent() for the window and store the new size whenever it changes so that you can track back to the previous size when needed.
Already tried that, but it's difficult to deal with window states, because resizeEvent is normally (or always?) called before window state is updated and it seems like when window was maximized via showMaximized it doesn't always have Qt::WindowMaximized flag in its state (maybe somehow related to previous geometry). I thought normalGeometry would return what I need, but it seems to return the same result as geometry does.

Anyway, thank you.