Photoshop maybe easier now that it's subscription based but I'm not sure. I still use CS6 that I bought on disc years ago. Anyhow, I think you're going to be happier with a 15" screen. I have both and prefer the 15" when editing. Now with that said, the 13" is slightly easier for traveling...smaller and lighter.
You could probably get a more for your money buying PC but Mac does have some nice specific programs. I'm really digging Final Cut Pro X for example. Anyways, I would consider 16 GB RAM if all possible. The storage isn't too bad, you can always buy an external portable hard drive. I take a 2TB or 3TB portable HD when I take my 13" due to it's small storage.
Definitely planning on bringing a hard drive. Though, I do have plenty or storage in OneDrive.
Not sure about the screensize. Let's find out! I'll probably visit the store today and take a look.