What external screen size/resolution will make using Xcode on an 11 inch Air better?
I would look at the pixel counts for the current iMacs since I've never heard of anyone that couldn't get substantial work done with the smaller but that most developers really appreciate the pixels of the larger iMac if they do substantial coding (6 hours a day or more).

http://www.apple.com/imac/specs/

The lower pixel count is 1920-by-1080 and the larger is 2560-by-1440

I've actually been productive on the 11 inch Air when traveling, but having a second display of any size was a big plus when at a desk to have code or documentation on the built in display and the app running on the larger screen (or the other way occasionally). 

The fact that you have one use in mind for the Mac means you probably can do with less screen than someone that will also want to do communication, email, calendar as well as coding.