Large, 50 inch by 40 inch maps are plentiful in the London Underground.
I bought a full-sized copy of the London Underground map, laminated in plastic, at the London Transport Museum gift shop in Covent Garden. I’ve never seen the full-sized maps offered in their online gift shop (http://www.ltmuseumshop.co.uk/), only smaller, poster-sized versions. The online shop does have the shower curtain version, though, which is actually larger than the full-sized map (and omits the advertising at the bottom and top right corner). I got that shower curtain as a thoughtful gift in 2009.