Is the problem that the backplanes online that you can see are too large? It could be the case that small backplanes are either rare or nonexistent. As you've noted, many computers of this era were large - cases tended to be massive with lots of extra space, and people didn't even need all the ISA slots that came on their motherboard (modem, sound card, video card...that's it) - let alone have need of a backplane that would accommodate more.