yeah Hailey, they are not the same size. very few companies make cd decks the same size that GM made our factory units (din and a half) and most every after market decks are just a single Din size. pioneer makes one that will fit perfectly, so does audiovox.
Heres a link to the ones that will fit with no modifications.
these are the only ones that i have found so far, but if you dont decide to go with these ones, and pick one that you like...crutchfield will supply you with the kit that will center it into the dash for free i believe. lots of people here with this done, just look around.
If the link doesnt go directly to the page, just search for GM/Chrysler-sized Receivers