Same issue, the start-maximized and windows-size solutions didn't work here
To make opera remember to stay maximized when I close it, I had to change the
[Settings > On startup]
back to:
"Retain tabs from previous sessions"

If I change to either of the 2 other options (begin fresh... or Open a specific...) then when I close > reopen Opera the windows go back to that small windowed mode.

Ubuntu 22.04 LTS
Opera 88.0.4412.27