Although whatever block you get has to be big enough to convince people to route to it. Too small and it won't matter. No ISP wants to carry the load for millions of tiny IPv4 blocks.

The smallest you can get is a /24, and everyone accepts routes for that.

In 1997, I remember running 2 T1's w/BGP on a router with a whopping 32 megs of RAM. I think there were 50,000 routes! How things have changed...