If the largest 32 bit number I can express is 0xFFFFFFFF, then in ascii representation, is this 64 bits in size (in hex)?
Yes, assuming you use one octet per ascii character, as is conventional. However you might also need a terminating nul, and maybe the "0x" prefix. Also you can use a 7 bit representation for ascii, it's still an ascii encoding (though hard to work with on 8-bit based platforms).