@gigara: if it's any consolation, I'm experiencing the same thing and your example mirrors my own. I believe the issue is being worked on but I have not heard any update on this in several weeks.

thank you. i manage to fix it temporarily with zoom from settings but now i can't zoom in-out in pictures and interface is still tiny. text size also not working on most website. i guess we have to wait untill HiDPI Support is fixed because that is best way to fix this issue.

my friend has 1440p(3k) computer i can't imagine that resolution.
Size is only an issue with dpi between 100 & 175. 200 is switched off. So the 3k monitor is "fine" (blurry, but not tiny).

sorry i didn't quite understand dpi part so it doesnt support 1920x1080p ? then i guess we have to wait untill its supported otherwise its really annoying but still i love opera