It’s tough to say. The portability of the Air is absolutely amazing, but I find the 13″ screen can be a little small at times. I think you should get a computer that is best for the *majority* of your situation. So if you are designing at home for 90% of the time, get a 13″ Air and hook it up to a 27″ monitor.
If you are on the road more than you are at home, then perhaps a 15″ is more up your alley.
I used to plug a laptop into a large monitor but then I just thought “whats the point” and bought a desktop. If im going to use a laptop I want to USE the thing in its entiety. Otherwise I’ll get an iMac.
My train of thought is headed towards no other option but the Retina Mac Book Pro due to its insane resolution in a small screen.
I’ve got the 13″ MBP and I can say it’s enough to do web design on, but not full time. It’s just a supplement until I can get back to my office. Like TheDoc said, you’ll need to determine how much of your development you are doing at one location or are you going to be traveling around all the time.
If I was doing 50% or more on the laptop, I would probably not use a 13″, and even 15″ is tough. But if you are doing maybe like 25%, then get a nice 27″ monitor and use that when you get back. 13″ isn’t straining by any means to work on, it’s just far less convenient than my 27″ I have at my office.
I’m also in the midst of this situation and have determined I’ll be getting a Mac Book Air.
I have a 27″ iMac in office where I spend 90% of the time so that’s great, but mainly want a portable Mac to use on the move and also on the couch/in the garden. So probably going to grab an Air and an additional Thunderbolt Display when I buy.
From what I use, Sublime Text 2, MAMP, Codekit, Tower and Codebox an Air should be able to handle everything no problem.
If you need to do more stuff than that, like image editing etc it might be worth the power and screen real estate to get a Pro.