DavidR - I did try to play around with those settings - without success , and I put them back as they were .
But get this - just did a Restart - and presto the window has a normal size again .
I thought , an update might have been done automatically - don't know what else could have happened .
Any change in those settings I would guess would require a restart to come into effect.
I have no idea if it is/was related to a program update, if it were I would have expected many more topics/posts on the subject. However, why this would happen at all after working fine previously is beyond me.
Since I'm active in the forums (by not an Avast Team member) I see notifications of new Program update releases, I have program updates set to manual. I look at what has changed, does it impact how I work and use Avast and should I update then. Sometimes I don't, such as in this case.