And the reason they were modeled after the dollar bill size is because there were already many types of systems for storing and organizing them. That came in handy for the census.
replyThe old BBC Connections series has a segment with James Burke using the old census tabulators.
In the end, all reasons resolve to either "it's what we had at the time" or "someone thought it looked good."
reply"Everybody just liked it that way and it costs too much to change it now": https://www.exocomics.com/743/
replyNot always, for example original CD disks had capacity of 74 minutes to accommodate Beethoven's Ninth Symphony.
replyThat one also turned out a myth :) CD size was determined by Cassette tape dimensions (diagonal, human can still hold one in one hand) and that combined with conservative pits/lands/track pitch choice drove the play time.
replythus CD runtime was derived from something "what we had at the time".