I downloaded a MetaHuman with 8K textures in Unreal Engine 5.6, but in the texture folder it only shows 2K textures. Where can I find the 8K face texture?

This seems to be true whatever setting you put in the assembler panel. However, after assembling, stay in the Metahuman Creator (MHC) and go to the Metahuman Character menu (top centre left) and there you can download the full size face textures - not body though - don’t know why. You can simply export, add to the same texture folder and use that then instead.