I am also developing a QuickLook extension, and still have no idea on how to resize the "window". It should be possible, because I installed apps like Sketch or Pixelmator Pro, that include the QuickLook .appex, and they successfully resize the window to fit the content, wether it is smaller or bigger than the default size. I can't yet find an open source project that handles this. Also, it seems that the system image QuickLook previews and others are written in the legacy .qlgenerators (found in /System/Library/QuickLook

I couldn’t. It seems that is intended for security reasons. That way you could read personal information out of files and send it somewhere else. But I cannot find any quote on this.