Yes 13” is only for some emergence things like server reboot when you out of office or some quick site fixes. 15” ideal for Apple notebooks (with 1440x resolution or retina). I don’t recommend to use notebooks with 17”+ its very heavy and not confinuent to use and take with you. For office use I absolutely recommend you 27” iMac, you can save money on CPU or HDD, but don’t save money for size, don’t take 24” iMac, 27” is ideal size.
I have to disagree with you, massively.
I have a Macbook 17” from 2010. It might be a little on the heavy side but it’s not that much of a problem. In fact I’ve stopped using my desktop and this is now my sole machine. The weight difference between a 15” and 17” isn’t an issue when I’m out and about, it’s entirely a marketing ploy. If it’s heavy your muscles will grow and get over it.
Furthermore the OP is looking to buy a mbp/air.. why are you talking about iMacs?
I recommend you to buy notebook with Windows.
You will not be less professional if you don’t use apple products.
wrong attitude completely. The correct one would be buy a Macbook of some sort and bootcamp Windows on it so you can develop for and can function on both platforms.