How short do you dock a tail?
It all depends on the dog, your vet will know.
It is against the law to dock a dog's tail in Australia and the UK. If you do dock a dog's tail in Aussie or UK you can face fines up to $37,500. However in the UK dogs that are classed as "working dogs" at the moment can have their tails docked by a vet - however you must prove the puppy is a working dog.
Medically, no, it is not necessary to dock (snip or shorten) a bulldog's tail. However, breed standards in the US and possibly other countries require a bulldog's tail to be docked for show purposes, and most people who own a bulldog will have the tail docked. No. According to the AKC breed standards, a purebred English Bulldog's tail isn't docked, it's naturally short.