At least 4 time the size of the bunny is the rule of thumb, but the more time you want them to spend in the enclosure the bigger it should be.

Here's a great site about rabbit enclosures. At least this is the kind we have. Easy to make, easy to adjust in size, and lotsa room. Karen made ours in one day. I really need a newer pic of it. But yeah, that is the one thing rabbit.org is missing... Pics! This is a bunch of owner made sites about their NIC (neat idea cube) cages.

http://homepage.mac.com/mattocks/morfz/nic.html