First of all, your images are automatically considered copyrighted at the moment of creation. You can register them at the copyright office but it's not mandatory but offers more protection in a lawsuit.

As for actually selling your photographs, you have to consider ways to get people who might be interested to see them. Local exhibits, maybe displaying at local restaurants or on a web site would be a few ways.

Enlarging an 8 megapixel image is no big deal. Your limitations are based on image quality, subject matter and what the use of the image will be. You do need a program like Photoshop that will let you fine tune the digital file when preparing it to be enlarged.