iOS app icon size: 1024 by 1024

This is a master rather than a size that ships as is. Every icon the operating system draws, on the home screen, in settings, in search results, is derived from artwork at this resolution, which is why it is worth building it as a flat square with no scaling artefacts rather than as an assembly of smaller exports.

Two conventions catch first time submitters. The corners should be left square, because the system applies its own corner mask and a pre rounded icon ends up with a visible double curve or a pale fringe. And the artwork should be fully opaque: transparency is composited against an unpredictable background, so a logo on a transparent field can end up invisible on a dark home screen.

1024 divides cleanly by two four times over, which is why the derived sizes are all powers of two or simple fractions of it. That makes downscaling from this master well behaved: each step halves the linear dimensions exactly, which is the case a resampling filter handles best.

For producing the file, the image resizer accepts the exact dimensions and outputs PNG, and the favicon generator covers the web side of the same problem with its own set of sizes plus a real .ico. Neither applies a corner mask, which for this canvas is the correct behaviour.