Records for pubs is quite a difficult subject to research as there are so many different pubs and determining what is a pub can itself be difficult. Is a beer festival a pub? If so, the largest pub would probably be the Great British Beer Festival. Also, a beer festival is likely to hold the title of longest bar.
Determining the oldest pub is also a very difficult process and one that will probably be never proven.
That aside, here are some pubs that can be classed as 'one of' the oldest, biggest, smallest, etc.
Oldest Pub
The title for oldest pub is a hotly contended title and one that is quite difficult to prove. Guinness World Records lists the Ye Olde Fighting Cocks which is a 11th Century structure built on an 8th Century site.
Many old pubs in Britain have been rebuilt once or twice and the Ye Olde Fighting Cocks is no exception. The pub was rebuilt in 1485 but according to Guinness World Records the pub was not moved to another location. The Man and Scythe disputes this and such claims it is the oldest pub dating to 1251. But, that pub was rebuilt in 1636.
Another contender is The Ferry Boat in St Ives, Cambridgeshire claims to date from 560AD but has only been proved as far back as 1100. I could go but instead you can visit this site which has more info on old pubs.
Incidentally, the oldest pub in Wales is the Skirrid Inn located in Abergavenny which can be traced back to 1110. As well as being a pub, it was also used as a courthouse where people were hanged inside the pub. Unlike the other pubs I've mentioned, the Skirrid Inn has stood for nine centuries.