In GNOME 3.14 we show any valid application in the software center with an application icon of 32×32 or larger. Currently a 32×32 icon has to be padded with 16 pixels of whitespace on all 4 edges, and also has to be scaled x2 to match other UI elements on HiDPI screens. This looks very fuzzy and out of place and lowers the quality of an otherwise beautiful installing experience.
For GNOME 3.16 (Fedora 22) we are planning to increase the minimum icon size to 48×48, with recommended installed sizes of 16×16, 24×24, 32×32, 48×48 and 256×256 (or SVG in some cases). Modern desktop applications typically ship multiple sizes of icons in known locations, and it’s very much the minority of applications that only ship one small icon.
Soon I’m going to start nagging upstream maintainers to install larger icons than 32×32. If you’re re-doing the icon, please generate a 256×256 or 64×64 icon with alpha channel, as the latter will probably be the minimum size for F23 and beyond.
At the end of November I’ll change the minimum icon size in the AppStream generator used for Fedora so that applications not fixed will be dropped from the metadata. You can of course install the applications manually on the command line, but they won’t be visible in the software center until they are installed.
If you’re unclear on what needs to be done in order to be listed in the AppStream metadata, refer to the guidelines or send me email.