I've visited some pretty big bookstores in, for example, London. Foyle's (in Charing Cross), Dillon's (near the U of L) and Waterstone's (near Picadilly) are three that come to mind.
Bookstores I've visited that appear on various 'top ten' lists are Borders in Glasgow (lots of space) and City Lights In S.F (very cramped).
The list of great bookstores that crops up over and over on the net mostly includes bookstores in cities, Lisbon, Buenos Aires, etc. that I haven't visited.
Good luck figuring out which is the 'largest bookstore in the world' through Google. Not that I think bigger is better or biggest is best. But for selection and the pleasure of spending time there Powell's is one of my favourites.
The so-called World's Largest Bookstore, in Toronto, probably isn't. I've been there lots of times. It has nothing on any of the stores mentioned above.
Questions of size aside, some of my favourite bookstores are Book Soup in LA,, Eliott Bay Books in Seattle, Nicholas Hoare in Toronto and Munroe's in Victoria.
Got a favourite bookshop? Amazon doesn't count.