Plenty big. In any case, many sites are stuck assuming your screen is only 800px wide, or the generous ones go up to 1000px - far more narrow than the screen can support. Any wider and text flows so wide you can't easily read from line to line anyway, so all you really use is height, and everyone is used to scrolling. And vertically it's not much different than any other (widescreen) laptop.
I'd buy the 11" Air in a second if I had the money. Having the 15" MBP is handy for movies, but for almost everything else it's noticeably larger than necessary. If you really need the space, you're probably hooking your laptop up to a second monitor anyway, in which case the slightly smaller space is inconsequential.