I tried to open Opera (10.60, Linux) with the command line option geometry for setting the width and height of the toplevel window. (I wanted to make some screenshots with different window sizes.) It was a hell to get it running properly, but finally, I found a solution that works.

First, I was not able to open a new window with the geometry option but it is possible to open a child window of a kiosk window with the correct geometry values.

You have to open a kiosk window that is just a parent holder. (The following commands will be executed in a terminal.):

opera -k >/dev/null &

Secondly, you can open a new child window with the expected geometry option:

opera -newwindow -nomenu -nocontextmenu -nodownload -nomail -nomaillinks -nosave -geometry 1024x768+0+0&

Good luck.