@lh-mouse
I know it's an old thread, but hey.....
For me, two solutions are working:
1 - For MAXIMIZED window edit Opera launcher -->right click on icon/ Properties/Application tab/ field Command: opera %U --start-maximized
2 - For CUSTOM window size edit Opera launcher -->right click on icon/ Properties/Application tab/ field Command: opera --window-position=118,64 --window-size=1682,951 %U
That sizes are actual settings for my desktop.
I'm on KDE Neon (Bionic 18.04), Opera 56.0.3051.31
Cheers