Hi
Are you after a lop or another breed of rabbit?
The smallest rabbit would probably be the Netherland Dwarf the lop sizes are the mini lop, dwarf lop, german lop, French lop.
If you look on the BRC website - British Rabbit Council they have listed all the different breeds with show weights listed.
The best thing would be to visit a local show, have a look at the different breeds and talk to the breeders. Again all the shows are listed on the website. If you need any more help or info just let me know.
First picture Agouti German Lop, second picture Agouti Mini Lionlop - the german weighs about 8 lb with the lionlop 3 and a half pound just to give you an idea of sizes and weights.