I don’t think you can fully separate out the internal design from the external dimensions. And a lot depends on the volume, pressure and temperature of the air/gas behind the projectile.

In general, the more diameter you have, the better. It appears easier to build an effective short fat suppressor than a long thin one.

But long thin ones can be effective. The well-known Parker-Hale rimfire one (also used on airguns) is a very old-fashioned and quite crude device. But it works well for its intended use.