You can try the --force-device-scale-factor command-line switch.
On Windows for example, you can hit Windows key + r to open the run dialog, type cmd and press enter to open a command prompt.
Then, with Opera closed, you can type:
"%localappdata%\Programs\Opera\opera.exe" --force-device-scale-factor=1.5and press enter to start Opera.
1 = 100%, 1.25 = 125%, 1.5 = 150%, 2 - 200% etc.
Then, if that works, you can modify Opera's shortcut to do that.
For Opera's pinned taskbar button, right-click it, right-click "Opera Browser", goto "Properties", switch to the "Shortcut" tab and add a space and --force-device-scale-factor=1.5 to the end of the value inf the target field, click apply and OK out.
You can do the same thing for Opera's desktop shortcut, but right-clicking it and going to properties. For Opera's start menu shortcut, you can right-click it, choose "open file location" and then edit the properties of the Opera shortcut there.
If you want it to be a separate shortcut, you can right-click on your desktop, goto "New", choose "Shortcut", browse to opera.exe in its install folder, click "Next", name the shortcut "Opera Big" and click "Finish". Then, use that shortcut to start Opera.
If Opera is ever started by a file or protocol association though, Opera will open normally unless you edit some commands in the Windows registry. I can explain how to do that if ever needed.