The number of bits in the length of the word should be a power of 2; thus the next logical 'full word' after 64 bits is 128 bits.

    2 to the n...
    0 = 1
    1 = 2
    2 = 4
    3 = 8
    4 = 16
    5 = 32
    6 = 64
    7 = 128

For large files it probably doesn't make sense to worry about the size of metadata. For smaller files, particularly ones that haven't changed recently, it might make more sense to use some kind of compression scheme.

A decade later, we have a lot of decent compression schemes out of patent and, likely based on their now public methods, a revitalization of development towards schemes that are more optimal for different use cases. Some combination of pre-filtering stages and compression for large caches of small files (like a directory of source code objects or configurations that are infrequently read) might make sense today.