Well I am trying to find a compromise between not giving window mode at all, and giving 2 window mode options (1280x720 and 800x600), plus of course fullscreen.
I understand that there are people with 4k monitors, but they can still play fullscreen (with pixel effect) and I believe they are still a minority anyways.
I think as a usability point of view, not giving the possibility for users to play in a window is very bad, but this is just my opinion of course.
As you suggested, having the possibility to resize the visionaire window at original resolution would be ideal, but I don't think it's possible right now (I would be happy to be proven wrong though)
Anyway, given the constraint (I am not willing to give up window mode) any hint would be really appreciated