I used to work at Apple retail, and here's how we would help you:
What will you use this Mac for? Is your primary use video editing, Photoshop, video games, or music production? Get a 15 inch with dedicated graphics card (and don't upgrade the processor unless you'll be using it for more than 3 years for any of said functions). Is your primary use internet, email, documents, and/or photo organization? A 13 inch (possibly even a Macbook or Macbook Air) will do just fine. For either the 13 or 15, max out the RAM if you plan on keeping your computer 3+ years, and upgrade to the largest HD you think you might possibly use. Tons of photos and videos? Look at a 1TB or larger. Hope that helps! 🙂