As far as I understand, the VideoPlayer doesn't know anything about where it is, but rather just expands to fit within the given space the best it can.
I believe what you want to do is use a RotatedBox
as a parent of the video and set the rotation value. Depending on how exactly your app works, you may want to have the video player start horizontal and small, and have a full screen button that switches to landscape mode. However, if the app itself is set up to rotate you'll have to take that into account and un-rotate the video once the phone has been rotated horizontally, which will probably result in uglyness in the UI as the flutter rotation happens and you un-rotate the video.
You probably also want to use a dialog to show the video full-screen so that you can dismiss it and get back to the screen you were at.
I could probably provide a bit more guidance but it does depend on which way you go with that (either locking the app to portrait mode and doing the rotation manually) vs using flutter's built-in rotation.
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