Who knows what the base SSD size in the new MBP will be? I suspect it will be 128GB, but it maybe 256GB.
I have an iOS app (audio application) in the App Store. Besides that app, I did variations of the app to test things out, I have an OSX version for my own use, I have a couple of networking and other libraries (GNU source code), plus a bunch of example programs. Some graphics are used in the program, but not much. All of this takes up about 450MB. The Xcode.app directory on my computer takes up 4.7GB. There's also a directory in my home directory, ~/Library/Developer, which takes up over 10GB. That may vary - I may have a bunch of different old simulators and other stuff in there that may not be there if you're starting out fresh.
If you use graphics and other "assets" for your programs, the space required can expand considerably. If you use vector graphics, you won't use much space at all. So if you really do only have the requirements you listed in your original post and if the assets required by your program aren't large in size, you can get by with 128GB. But you should consider if it's acceptable to use an external device, be it a drive or SD card if you need it in the future.