It won't necessarily change the size of a print, but it will affect the resolution. If you make a print at the same size from two images, one of which is smaller than the other because it has been cropped, it stands to reason that the print made from the smaller image will have less resolution in the print because you started with fewer pixels.
However, if you want to maintain the same resolution then, yes, the cropped image will produce a smaller print.
As to cropping, there are two ways. You can make a selection using the rectangular marquee tool then go to edit>crop. Or you can use the crop tool. The crop tool is slightly more flexible as you can move it around, and change the actual selection before you commit to the crop.
Either tool can be used freehand, or you can enter fixed dimensions/ratios as required.