Waterloo is the largest, in terms of area (24� acres). That's according to Network Rail, who ought to know ;-)
http://www.networkrail.co.uk/aspx/959.aspx
Waterloo currently has 19 operational platforms but those platforms previously designated as 'Waterloo International' (for the Eurostar services which have now moved to St Pancras) will be added to Waterloo's capacity towards the end of this year.
In terms of current platform capacity, Waterloo is matched by Victoria, which also has 19 platforms:
http://en.wikipedia.org/wiki/London_Victoria_s tation
Liverpool Street and Euston both come close, with 18 platforms each.
St Pancras International only has a total of 15 platforms:
http://www.highspeed1.com/consumerpage.php?pag e=stp
London Bridge is often overlooked, when considering the size of stations, simply because it's not a terminus for many of the services which use it. It also has 15 platforms:
Paddington has 14 platforms and King's Cross has 11.
Clapham Junction has 16 platforms and, in terms of train movement's, is the busiest station in Europe.
Chris