Check out this list of huge bookshops around the world: https://www.bustle.com/p/12-of-the-biggest-bookshops-in-the-world-for-when-you-want-to-lose-yourself-in-literature-40100.
One of my favourite big bookstores was The Highway Bookshop located on Ontario Highway 11 near Cobalt, Ontario, which operated from 1957 to 2011. Considered a landmark and cultural institution in the region, it was one of the largest and most famous independent bookstores in Canada. At its peak, the store had over 300,000 titles in stock in a variety of locations, including the main building, on-site trailers and several warehouses. It has even had a book written about it: Highway Book Shop: Northern Ontario's Unexpected Treasure by Lois Pollard.
But if big isn’t to your taste, perhaps you can visit the world’s smallest bookstore: http://weeklypacket.com/multimedia/photos/2015/aug/20/worlds-smallest-bookstore/#.WLc-gn-RI17.