Tnx for the reply.
I tried something similar: I logged onto a different account on which I hadn't previously used PixInsight - and it works correctly (except I still can't change vertical size by grabbing the top of the window in windowed mode).
BUT, I copied the hidden .conf file from this account to my original and we seem to be back to normal. There are parameters that set window size and they seem to be set during installation(?) - it isn't clear how it can be changed from within the program so it [perhaps] can be set only externally. Anyway, I seem to be back to normal. I hope...
Regarding your suggestion, I see my "fullscreen at startup" is not checked by "maximize at startup" is - I wonder what the difference is...I'll have to play with that and see what happens.