Yes - Im the administrator
Windows 7 64bit
Opera 64.0.3417.83 64 bit

Where I find settings?

Open Opera's Preferences file with a text editor and find the following segment -

"browser": {
"maximized": true,
"maximized": true, a lot of times "maximized" is set to false.

I saw your mp4. I did it at morning when I had sure that opera after night opens not maximilized. I did it and it didnt help it.

I did it exacly and didnt help.

Where can I change it?
Where can I find it settings?

@nvmjustagirl: Your way with ctrl doesnt works

I reinstalled Opera and installed the newest version but it doesnt help and I still can not set Opera as a default browser. I press Use it but it doesnt works.

@nvmjustagirl : Im testing your way

@leocg : I did it.
New profile it means?

No, it doesnt works.

Now Ive got second problem - every time when I open Opera ask me: Set me as the default browser. I click set but it doesnt works.

I read it but it wont help - Youre writing about move window on left or right side and Im looking for any help to always open my opera on whole screen

https://forums.opera.com/topic/35794/solved-opera-opens-most-always-on-half-screen-why

The issue still exists and I dont know how can I fix it.

I installed the newest Opera (64.0.3417.61) and problem disappeared for now. If wont help I'll make test. Thank You