What size SSD would suffice for basic software-development / productivity?
A 128GB SSD would have enough room for what you list, with room to spare. A 256GB SSD, imho, would not cost that much more, and would give you more flexibility. 

Be aware that the version of Xcode needed to develop for iOS 7 requires Mountain Lion, and ML may not support that machine. For learning or developing for iOS 6, Lion/Xcode 4 should be fine.