Thinking About Buying a Macbook Pro for work( Web Design)

I want to by a macbook pro for the internship I am about to start for a web design firm. I was hopping someone could give me an idea of what kind of specs I should look for. I thinking I absoultly have to have 8gb of ram. But I'm on the fence about whether to get a 13inch or a 15inch screen. And as for the processor, I was hopping someone familiar with this kind of work would be able to tell me what kind of processor and I should get. What about upgrading the graphics card? Any advice would be greatly appreciated.