I tried to edit the post above for clarity but time elapsed and I was no longer allowed to do that.
Basically, if you are using Mint Cinnamon, all you have to do to have Opera open in any position and size you wish is to permanently pin a tab. It can be any tab you wish.
Edit the opera.desktop file at local/share/applications at the sixth line, under [Destop Entry], to this:
Exec=opera %U
and save the file. Log out of your computer and back in and it should work fine. Just make sure to have a tab pinned.