The great majority of vehicles have a 2" tall (or "DIN-size") radio; some Chrysler and GM radios are 3" tall (DIN-and-a-half), and some Japanese vehicles have 4" tall radios (double-DIN). Many vendors make 3" and 4" tall aftermarket radios, or you can purchase mounting kits that allow you to mount a standard (DIN-size) radio in these larger dash openings.
Another thing for you to consider when trying to determine whether or not the Grand Am stereo willfit you car is the electrical connections.
Oh, by the way. What Cass has told you is, the radio will fit in the hole. But doesn't know if plugs will plug together.
I have wireing for Grand Prix but not the Grand Am. May have to wait for Big-L or call dealer tomorrow.
Again,
Good luck