The numbers "3072 x 2304" are the number of pixels. So, it would give you an image file that is shaped like a rectangle, which is 3072 pixels wide by 2304 pixel high. that's a lot of pixels, and it's useful to have that much resolution if you are going to print the file in a large size, or crop to look at a particular detail. The downside is that the files are larger, you can't hold as many on your card, and they take up more room on your computer.

A3 is, roughly 11" x 17" That's pretty big. You probably don't have a printer that will print that size. A4 is your standard 8x10" photo.
here's a description with more detail than you need:

http://en.wikipedia.org/wiki/Paper_size

Professional/magazine quality prefer 300 dots per inch. So, 3000 pixels would be 10 inches. But- on a decent ink-jet photo printer, images look fine at 150-200 dpi. so, a 1400 x 1000 image, which is pretty small by todays standards, will actually make a nice 7"x5" print.