Hi, Laura. Guess what. The windows that they sent you are the right size, at least if I am reading your post right. There is supposed to be a half inch of clearance between the rough opening and the window frame. This is to allow you to install the windows level and plumb. Your rough opening may not be level and plumb, in fact it probably isn't. So what you do is go to the home center and pick up a couple of packages if shingles/shims. The shims are about 1-1/2" wide.
You start by placing the window in the opening, put a level on the sill, insert one shim from each side at each end under the sill and level it by sliding the shims. Be sure that the window is also centered in the opening from side to side. When everything is level, you can nail the bottom. Then you do the sides the same way, being sure that they are plumb.
Laura, you really might want to consider calling in a pro to show you haw to install one window. There are many different kinds of casing designs and the process is much easier once you see it done. You do need 2 people to install a window, one on the inside and one outside.