I would like to know what is the maximum size that an vim or emacs editore can open & edit
The documentation just states this:
Maximum file size 2147483647 bytes (2 Gbyte) when a long integer is 32 bits. Much more for 64 bit longs. Also limited by available disk space for the |swap-file|.
so "much more" will most likely run into your hard drive limitation since it goes to exabytes of alocatable size.