At the moment the appstream-builder in Fedora requires a 48x48px application icon to be included in the AppStream metadata. I’m sure it’s no surprise that 48×48 padded to 64×64 and then interpolated up to 128×128 (for HiDPI screens) looks pretty bad. For Fedora 28 and higher I’m going to raise the minimum icon size to 64×64 which I hope people realize is actually a really low bar.
For Fedora 29 I think 128×128 would be a good minimum. From my point of view the best applications in the software center already ship large icons, and the applications with tiny icons are usually of poor quality, buggy, or just unmaintained upstream. I think it’s fine for a software center to do the equivalent of “you must be this high to ride” and if we didn’t keep asking more of upstreams we’d still be in a world with no translations, no release information and no screenshots.
Also note, applications don’t have to do this; it’s not like they’re going to fall out of the Fedora — they’re still installable on the CLI using DNF, although I agree this will impact the number of people installing and using a specific application. Comments welcome.