Smallest prefix that can be meaningfully used is /24 (you cannot advertise smaller prefix), but I think that everyone advertising random /24s would lead to even larger costs for router upgrades than IPv6.
I think there are still bigger than /24s where /24 announcements aren't allowed. People filtered for a while (at least around 2000-2002) in those ranges, so you couldn't just take a random swamp /16 and turn it into a bunch of /24s and expect them to be visible to everyone, although it did work to most. I haven't paid much attention to routing politics for the past 5 years or so though.
