The largest eye in the world belongs to the giant squid, which has eyes up to 27 cm in diameter (some reports from the 1800s say 40 cm). Extinct ichthyosaurs also had very large eyes, 20-30 cm in diameter. How large can eyes realistically get? What evolutionary pressures could result in obscenely large eyes?
Oh, those sexy eyes: You need to have some advantage to having eyes, or else they degenerate like blind fish. So why do animals get distorted into seemingly impractical forms?
- The most obvious thought is that if eyes become a sexual display somehow, the might grow to the point of being dysfunctional. If the opposite sex likes it, it will select for it.
- Big eyes imply dark places - either nocturnal animals or ones living in dark places like the deep ocean (those giant squid). This is a decent link to the whys and hows. https://animals.mom.me/nocturnal-animals-large-pupils-7471.html
- Some special selective pressure and nurturing could result in a specialized species with really big eyes. If an animal (say, an owl) were domesticated and selected for large eyes because they can see ships at sea (for example) and alert their owners, those with the freaky birds can flee threats (or pursue victims, for pirates) even at night. People are people, and for something like this, big eyes could be selected for even if they don't actually result in better vision (appearance creates reality).