@ perknhThought you might find this interesting ...

I've been busy, building my
Dell E6430 Peppermint/'10' 'Road Warrior' lappy. I need to have it finished by the first week of May, for a maiden voyage to Vegas.
I ordered an
OEM backlit LED keyboard yesterday. It should be here in a couple of days - and that'll be the last hardware upgrade I'll be doing to it.
On the software end, I've tried three different vers of Opera, and truthfully, the only one I like is the dev release - currently at 46.0.2567.0. I like it so much that I purged Chromium and Firefudge from this machine.
Anyway, I just got done doing the 'window-size' trick in Opera-Developer, and it didn't work - just like you. So, I tried twice and thrice, and still no difference

Then, it dawned on me that I was clicking the Opera button that I had placed on the panel, last week, pre-haxor. When I tried the Opera button in the Whisker Menu, it resized just fine. So, I removed the Opera button from the panel, and replaced it with a new copy from the Whisker Menu ... Bingo !

I wonder if that's the same problem you've been experiencing. Which button were you clicking ?
BTW, I added the 'Internet' category to the Opera-Developer .desktop file, just to be sure. Here's what it looks like:
[Desktop Entry]
Version=1.0
Name=Opera developer
GenericName=Web browser
Comment=Fast and secure web browser
TryExec=opera-developer
Exec=opera-developer --window-size=1100,739 %U
Terminal=false
Icon=opera-developer
Type=Application
Categories=Internet;Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;application/x-opera-download;
Actions=NewWindow;NewPrivateWindow;
[Desktop Action NewWindow]
Name=New Window
Exec=opera-developer --new-window
[Desktop Action NewPrivateWindow]
Name=New Private Window
Exec=opera-developer --private
Okay, back at it ...
