Thanks Devlin, It Fixed it.

What I did...

sudo nano /usr/share/applications/opera.desktop

and followed your code replacements...

Find the line {code}Exec=opera %U{code}

And replace with {code}Exec=opera --force-device-scale-factor=1 %U{code}

Restart opera and viola!

I have Opera on the Ubuntu panel and it started with small text. But from the Application Menu, Opera started with the correct font size so I removed the Opera from the panel and replied it back to the panel and all is good again.

I too have the same problem after I updated to Opera 43.0.2442.806 on Ubuntu 14.04.5 LTS (x86_64; MATE). Having a hi-res monitor makes it very difficult to read Opera's Tabs and Menus. I didn't have this problem before this update.

I hope it gets fix soon or I'm have to dual-boot to Windows 8.1Pro which doesn't have this issue with Opera.

Thanks for a GREAT Browser!!