I just finished my freshman year at a University with a 15" MBP. I don't mind the weight, the screen is fantastic and having the graphics card is nice every now and then.
That being said, if I had to do it again, I would absolutely pick the 13" Air or Pro. The SSD is nice, but nothing I do for my degree really would show the speed, so I'm absolutely fine with a mechanical drive. I use an external monitor in my dorm, since any extensive project demands more screen real estate. Writing code on that 11" Air will be one of the worst experiences, unless you have great eye sight and small text isn't a problem. I will say that I wear contacts for my vision and I had a hard time reading code on my friend's 13" Air.
On the point of needing Windows: I ultimately haven't yet, and I know plenty of other people who haven't through their Junior year. Yes, I did use it to write Assembly for a class because I wanted to use the MASM assembler and Visual Studio, but plenty of people got by just fine using OSX and Linux only. All of the intro classes at my university were done in Python, and most other universities use either that or Java, both of which are ultra cross platform. C and C++ are also cross platform and very well supported by Xcode.
Battery life wise: Only my CS classes were in rooms with power outlets around, most other lecture halls were pretty old. Granted, I only needed my laptop for CS courses, and I was good for around 4 hours with the keyboard backlight off and screen brightness turned down. I also always had a charger with me, and I was able to charge it a bit between classes if I ever needed to.
My suggestion would be to hold out first for WWDC and see what gets announced. Hardware releases don't typically come at WWDC, but there is speculation due to shortening supply of Airs. After that, 13" Air or 13" rMBP is my suggestion, but with maxed out RAM. You'll want it later. Don't get hung up on 11 vs 13 for fitting in lecture halls, I could sit with my 15" just fine in any lecture all I was in, and there were other people with much thicker Windows laptops that fit just fine as well.
When I started school the MBA wasn't really an option, and I ended up with a 15'' MBP.
Fast forward to now, and it is rocking maxed out ram and an SSD (Ignore the hype, make up even bigger hype in your head, then believe your own hype, it's that big of a deal).
One of my last courses as a software developer has me creating a 3D game. It's going to be a fun summer, but I noticed something when I got started.
There is a really good reason it says "Pro" at the bottom of this screen. If you're going into CS, I'd shoot for one of the Pro models. I know personally I'd love to walk around with a MBA but I also know that I can't drag my BeastMac (27'' iMac) with me to group meetings. Any time you see a professional working on a mobile mac, in any field, they almost always have a 15'' Macbook Pro. Apple doesn't change these things because they're built like tanks. You can push them as hard as a computer can be pushed (I've had this thing sit at <500mb of ram free, Maxed out CPU clock, and over 100C for almost 12 hours straight more than once), and they don't give out. Every ounce is used wisely. A quad-core Macbook Pro will last 4 years with EASE.
That's why the 15'' is such a great professional's machine. Even in it's low res form there's plenty of space to keep you from getting too cramped when you're on the go (You never know when a class will get canceled and you'll get an hour to hack). Although if I could go back, get the hi-res upgrade, it's worth every penny. My work Mac is a 13'' Macbook Pro and I find it nearly impossible to get anything done without an external monitor, although I admit the size is nice.
The 15'' gets INSANE battery life out of the box. I mean, I got my 2010 as a replacement to my 2009 model (it ate more than a few logic boards), and I remember when I first got it, it easily did 7 hours on a charge. Three years later at 650 cycles I still can get 5 hours out of it pretty easily.
I didn't know what CS was going to look like when I started school...computer wise. I didn't know that upgrading my gear every couple of years wouldn't be an option anymore, that I'd have more important things going on. I certainly didn't think I'd be building a 3D game at any point.
But here I am, and my WarMac is here too, jam packed with all the developer goodies I could ever need. I also typically carry a FW800 hard drive with my Time Machine backup as well as a Win8 VM, a Win7 VM, and an Ubuntu VM.
Plus the dedicated graphics card means I can play a quick game of Counter Strike when I get a little down time.
Trust me. If you're going to be a Software Developer, get a Pro model. Apple knows what they're doing.