I too have this problem, although it happens to me quite often; often enough that I have also loaded Chrome; as a back up for when Opera cuts out on me. BTW I don't run extensions so this isn't extension related.
Best posts made by kilroy-73
Opera constantly resizes itselfOpera for Windows
I'm new here, however, I have been having some issues with build 60.0.3255.70 constantly resizing itself every time I use the mouse or tab button to navigate tab/link/bookmark ect.... Normally, I have Opera open and set to take up the full screen (maximized) however, the way Opera has been behaving it doesn't stay maximized for long, it changes sizes anywhere from x4 my monitors size, down to just barely large enough to have the X close button. Would any of you folks know how I could go about fixing this? Opera devs have been zero help here at all.