Chad (user9) has a fantastic answer. My answer on minimum space comes from a different perspective, and is just different not better.
For the pet house rabbit there are two common housing scenarios.
The rabbit has full access to some bunny proofed area all of the time. The bunnies "safeplace" for napping and such is a cardboard (or something) box that cannot be locked.
The bunny has a "cage" that they are locked in during nap times and they are let out to exercise in a large area of the house a couple of times a day.
In either scenario a couple things to keep in mind
- Your rabbit is going to live 8 to 12 years, with spans of 16 and 17 possible.
- Your rabbit can run 30 miles per hour and would like to have room to do it.
- Insufficient exercise can be harmful
Scenario 1; If the bunny is going to be restricted to a designed area for all or the majority of their life. The smallest space is about 8 feet by 8 feet (2.4 x 2.4 m), this is two standard exercise pens connected together, or one placed in a corner. This provides room to make short dashes, and running "binkies" (happy bunny hope). Supplementing this with supervised access to a larger part of the house a couple of time a week, can make for long happy & healthy life. See Can house rabbits have free access to the whole house all the time? for more detail on expansion.
Scenario 2; In this scenario for reasons of safety the bunny is restricted to a smaller area during much of the day and night. Rabbits are crepuscular - they are most active in the morning and evening. This is one of the things that makes them excellent companion pets for people with small spaces, who work (or go to school) during the day and sleep at night.
In this scenario the bunny is locked in their "safe place" while you are away and they are sleeping. The space should at a minimum be one bunny wide by two bunnies long. If space is an issue, a smaller space can be used, with the addition of a second floor (but bigger is better). The bunny must have room to stretch out, and you need space for litter box, hay and water.
When you wake up in the morning, the bunnies get out, they get to run and roam in a large part of the house that is bunny proofed. They have lots of room to exercise and when it is time for you to head out for the day, you get their food pellets, call them to their room and they come running, the bunnies and the food go in the "cage" it is locked and secure until you return home. When you get home in the evening, they get out, run play and watch TV with you. When it is bed time you prepare their lettuce call them to their room, they come running, the bunnies and their food go in the "cage" it is locked and secure until morning.