Do you need a good dedicated graphics card? If so you want the 15" version with the NVIDIA gpu. The Iris integrated graphics on the other models is supposed to be good as far as integrated graphics I believe, but they don't come with the dedicated gpu.
After that it's more a matter of speed and memory size. A faster CPU will potentially make things run faster, which is always better but of course costs more. I'd say you want at least 8 GB RAM, get more if you think your programs will need it. The hard drive size is up to you, however much space you think you will need.
So essentially, if you need a dedicated gpu you need to get the 15" which lists it in it's specs, otherwise just make sure you have at least 8 GB RAM and a big enough HDD for your needs. I'd personally feel more comfortable with at least the 512 GB hard drive on the 13", or at least the 256 GB on the 15", but that's just me