There's a lot of occasions where I'm checking the specific iOS icon sizes, and I remember I made a document a few years ago with some references. However, there's been a huge amount of changes since then. So I decided to create a new reference document from the Xcode 9.0 beta.
It's hosted as a Gist on Github, or you can view it below:
I'm going to make an effort to keep this updated, and I'll probably create one for the other platforms as well. Anyway, I hope people find this useful!