Ok, it is indeed a question of minimum window size apparently. The size I want is approximately 927px, the size I get is 1014 pixels (according to my screen cap).
If I close the F&R + several toolbars + the ToC I still get 1014.
If I leave it at 1014 but move it to the left (so the preview is not blocking it) I can keep all the usual toolbars + ToC and F&R and the size / position is restored on open.
Means I have a bit less space for reference documents to the left of Sigil but I will learn to live with it, I guess.
Thanks very much to both of you for helping me figure it out.