I develop on a 13" MBair it's the perfect size. I would go with a 13" and then connect to a monitor at your main work area when you need side by side displays.

I develop on a 13" MBair it's the perfect size. I would go with a 13" and then connect to a monitor at your main work area when you need side by side displays. I use a 23" Acer monitor from amazon and a rain mStand, wireless trackpad and keyboard, It's a great setup for my main work station. Having a 13" air is great for working in other locations, outside and at a coffee shop.