Actually, there's not really any "standard widths" in Eurorack. You see 84 hp (and multiples thereof) a lot but that's because 84 hp is about the limit that you can jam into a 19" rackspace. But other widths you see include 60 hp (Moog), 104 hp, 126 hp, 140 hp (Uli), 168 hp (double 19" rack width) and 197 hp (from ADDAC, using 1 meter length rails). As a rule, most cab makers tend to stay with even-numbered hp amounts since the vast majority of Eurorack modules have panel widths that come out to even numbered amounts. But not all...
The real bottleneck on sizes comes down to power issues...amounts of headers on distro boards, power load capability vs. potential load from the modules, that sort of thing. However, there ARE ways to deal with those, plus you don't exactly have to use typical Eurorack supplies...with some poking around, you can find LINEAR power supplies that fit the bill. And yeah, linear supplies are worth the expense + weight, as they're quite incapable of spewing ultrasonic rubbish onto your DC busses, and they tend to tolerate inrush behavior better than switching supplies. No radiated fields, either, aside of the usual line frequency junk (which they're usually good at filtering out).