sgunhouse: ah thank you for explanation.

jimunderscorep: yes that's what I meant "maximized window".

For my problem:
-geometry:1366x750 does not work for me,
but --start-maximized work.

Thanks for you answers

How to start Opera in fullscreen,
I mean by that a window that takes all the space available (Like this ALT+F10 and not ALT+F11)
Currently Opera start with that opera %U (What is %U)
I use Linux(Manjaro).
Thanks