http://en.wikipedia.org/wiki/Film_format
there seems to have been several things going on
some of the sizes were based on whole plate sizes and some of them were based on manufactured plate sizes ..
the sheet film sizes it seems were linked to dry plate / glass plate sizes
so when manufacturers were making glass plates camera makers began to standardize
on sizes ...
and sheet film, is smaller than paper and plates because there were a zillion plate holders out there still in use
and people who started using sheet film just bought a little metal sheath that slid into the plate holder and converted it
to film ... when plates went by the wayside, there was no point in making new holders &c since everyone ( manufacturers, users &c ) had grown
accustomed to the sheets beeing 1/16th" smaller than plates ...
bigger than 8x10 seems to have standardized in both english and metric, but because there was never a mass-market business for ULF sizes
they were just varied and sort of based on 8x10 and 11x14 ( guessing )
... 7x11, 11x14 ( half ) and 5x7 is around-1/4 a 11x14
... 8x20 ( pano double 8x10 )16x20 ( quad 8x10 )
... 12x20, 20x24 and their metric cousins ...
... too many others to list and guess about seeing
whenever kodak made a "new" box camera that took roll film, they created a new roll film format
so the film was proprietary and users had to buy the film from them ...
probably the current roll film formats are just based on the old sheet film sizes that are similar,
or PAPER sizes used for enlarging on ...
and 6x6 cm, was probably just .. random like everything else seems to be
" 3x7/8 x 5 7/16 looks like a good format, lets make it and see if it sells ... "