Thanks to both the above posters.
DavidR I have checked the active window title bar font size, it is set at 8, so no problem there.
iroc9555 icon image stays as per my screenshot after rebooting.
A bit strange, but not a deal breaker, maybe one day it will return to normal in the same way it went low resolution

Thanks again.
PS all other taskbar icons are normal, ie as they always have been, only Avast is different.