Which Mac for Web development + iOS development I'm a Windows user, but I'm looking to get a portable Mac for web development and iOS development. 

I will be running sublime 3, Xcode and probably Nginx or XAMPP as my server. Would the 11-inch MacBook Air be a suitable machine? If not, what would be the minimum spec machine.
The 11-inch MacBook Air would more than suit your needs as far as specs go.

But, the screen size makes it hard for developers who are used to multiple monitors or windows side-by-side.

On my 11-inch Air, I normally use apps in full-screen mode and use gestures to switch between them quickly, but I sacrifice being able to have side-by-side applications.

You might consider an 11-inch Air with an alternate monitor (or more than one) for when you can sit at a desk and get serious.