A bottle at around 600ml should be adequate for two regular rabbits for everyday use over a 24 hour period, but always keep an eye to ensure it isn’t looking empty. Giant rabbits will need a bit more.
If you do have more than one rabbit, you may wish to have more than one bottle in the cage just so they both have access to water at the same time. Two bottles at around 300ml each should be ideal.
But you may wish to buy one closer to 1 litre to be on the safe side or for more than two rabbits. Another alternative is buying one not much more than 150ml and topping it up throughout the day so they always have a very fresh supply of water, but do be aware that this is only the best option for people who are at home with their rabbit throughout the day and can keep an eye on everything.
As a guide, a rabbit will drink 50 to 150 millilitres of water per kilogram of body weight daily. Every rabbit is individual, but it does mean that the common Dwarf Lop, which should weigh between 1.9kg and 2.4kg, could have between 95ml and 360ml every day.
Most are made from plastic, which makes them shatterproof and safe to have around your small animal. This will be very strong, unsquashable plastic however so they will remain strong and sturdy no matter how much they are used and handled.
Some glass options are available too which are great for anyone avoiding plastic, and they are easier to recycle or clean. But they can be heavier so you’d need to ensure it was really secure to avoid it falling off and shattering.
The big issue is the material of the drinking cap. This should be metal ideally, as then your pet will be unable to chew it either to access the water or out of boredom.
The majority of bottles will attach to your wire cage via a hook or wire ties. Some come with screw fittings where one part goes on the inside of the cage and screws tightly to the bottle on the outside.
The latter is usually only best for wire cages and not mesh outdoor huts. Whichever you think is best, just be aware that bunnies could try and chew any fixtures. If you have a particularly frantic chewing bunny, wire ties may be harder for them to access than the large screw caps.