I have the 11" and I find that it's not too small. Safari was a little small, but that was easily remedied by going in and setting the minimum font size to something a little larger.
Office documents can be zoomed a little bit, if you find it too small, but the 11" screen is wider than an 8.5x11" sheet of paper. Vertical space is lost when compared to a larger screen, but that's not a deal breaker, in my opinion.
As far as coding, I'd say that neither the 11" nor the 13" will be optimal. Passable, probably, but having more screen space seems useful. As others have suggested, an external monitor (and keyboard) would be useful. If money is an issue, look into buying a used monitor from craigslist or other classified ads in your area. You could almost certainly find a decent, used monitor for less than the price difference between the two models. Then, you have the best of both worlds, a large screen and great portability.