Reputation unit is /64 block, so if you want to see a 100 people ISP as one reputation unit, it should get a /64 block. But AFAIK today in practice reputation unit is a country.
I have a /48. The amount of "we have detected unusual activity from your network" messages I get from sites, when I'm reasonably sure the only activity coming from my network is my usual activity on those sites, suggests that they're using something bigger than /48.