In the documentation for ldlen and localloc the size type is described as "natural unsigned int". Although I have an idea what is meant (on x64: 64 bit unsigned on x86: 32 bit unsigned) I haven't found any documentation for it.
So is there an "official" documentation about the "natural unsigned int"?
The term natural refers, as you thought, to what is native on the hardware. Although it's not specified explicitly in a glossary, you can make it out from other uses of the word natural in the spec:
The native size types (native int, native unsigned int, and &) are always naturally aligned (4 bytes or 8 bytes, depending on the architecture).
or
autochar indicates a platform-specific representation that is “natural” for the underlying platform