You're wise to use caution when removing your mirror - especially given its size. You can use one of two methods. Both will require repairing your drywall after, but if done properly, should spare you injury or further headaches - and will leave the mirror in one piece in case you want to decorate with it later. The first method involves buying a length of cutout wire that's longer than the width of your mirror. Cutout wire is also called windshield removal wire because it's used by auto body shops to repair windshields. Apply clear packing tape over the mirror to minimize mess - and risk of injury - if it breaks. Then stretch the wire tight, and slide it between the back of the mirror and the drywall. With at least two other people holding the mirror in case it falls, wiggle the wire back and forth until the mirror dislodges.The second method requires a drywall saw. Push the saw into the wall near the edge of the mirror, and remove the section of drywall the mirror is glued to. This is the safer of the two methods, so go with this one if you're fully replacing drywall anyway. Again, don't attempt mirrir removal with a few strong hands there to hold the it once it's been released from the wall. Good luck!