The theoretical 64-bit address space is 2^64 or 16 exabytes (18,446,744,073,709,551,616 bytes)
Microsoft limits the maximum physical memory to 512 GB for Windows 8 Pro and Enterprise. (reference)
This is mostly for commercial reasons although I have heard that Microsoft will only authorise up to the memory size for which there exists equipment they can test.
As far as the per-process user mode virtual address space, this was limited to 8192 GB prior to WIndows 8.1. Windows 8.1 has increased this to 128TB. (reference) (reference)
The way the virtual address space is laid out is a result of the way that current processors only implement 256TB of virtual memory.