The smallest cup size is probably about a 28AA. Any smaller than that and you don't need a bra, though AAA lingerie does exist.
In the high street, the upper limit is probably a 48DD. Figleaves.co.uk and other specialist retailers sell up to a 56FF which boggles the mind (!!).
The (non-enhanced) breast size record is currently 56WW, held by American Norma Stitz and recognised by the Guiness Book of Records. Maxi Mounds (no kidding - guess how she earns a living!!), also an American, holds the record in the 'augmented' category.
The UK's biggest natural breast award is currently held by 18 year old Chloe Rogers, who wears a 34KK. Closest rival Rachel Aldana wears a 32 JJ. Remember, letters are more important than numbers!
Not that I'm an expert, but if anyone needs a judge and would like to make a claim I've got a tape measure lying round here somewhere!