@pointbob
Sorry, english is not my first or even second language 😞
In windows you can drag window to side of screen and then it 'snaps' to that side of screen - looking at you screenshot seems that you have Opera 'snapped' to right half of screen. Of course this may not be so 🙂
If my ideas didn't work, then sorry - in my setup additonal Opera windows are same size that original one, just located at a bit different position - I just can't duplicate your troubles. Maybe someone other will try also?