can i develop ios app in macbook air

App development itself, meaning the repetitive cycle of writing code, compiling, testing, debugging - is not a particularly CPU-intensive activity and the base MacBook Air configuration is sufficient. I strongly recommend upgrading its CPU to the i7 and 8 GB RAM. The storage space you require will be a function of what you are unable to offload to external storage so 256 GB may be enough for you.

I also recommend the 13" version for a variety of reasons, one of which is greater battery life. The smaller version seems too cramped for me and even 13" is not that large a display area, but that is a personal decision only you can make.

Yes you can use a MacBook Air to develop iOS apps. You will need Xcode, which is available from the Mac App Store. It is free.