When an instrument is popped out, the content of the window is drawn with a fixed aspect ratio. So for example if you pop out the G1000 PFD and then increase the vertical size of the window, the G1000 PDF content will still be drawn as 16:9 in the vertical center of the window.
While this would initially seem to make sense, it is entirely possible to need to adjust the size of a popped-out window such that the aspect ratio of the window frame is different from that of the instrument. In this case, popped out instrument panels should re-size to fit the aspect ratio of the pop-out window. This should either be the default behaviour, or an optional behaviour.
Overall the popout process should not rely on third party software. Asobo should make the panels configurable and assigned to certain screens. So if I start a flight in the C172 G1000 I would expect the popouts to show up on my external G1000 suite. Of course you are right with screen sizing. Therefore it must be configurable for scenarios like an airliner or turboprop where someone might want to popout more than one display to only one monitor.
Another innovation: I would like to be able to make a “screenshot” of a cockpit and have it shown on an external monitor. This way every aircraft´s instrument panel could be easily copied as a “popout” making it unnecessary to search for not available airmanager templates like for the new PC12.
Or to put it more shortly, the sim should have the same windowing and view behaviour as FSX and P3D