We have 2 bathrooms and both of them 'had' a mirror glued to the drywall. The biggest one was about 7 feet wide and 4-5 feet tall. The glue was like a black thick tar. My boyfriend was able to pry them both off by forcing the mirror to move up and down with just his hands (he never pulled away from the wall, just up and down starting at one side and working towards the other).
I was terrified as I watched, but I realized later that the process did not invove breaking the glue, but rather ripping the outer paper off the drywall. (the drywall was easy to fix up with drywall compound).
The big mirror was completely off the wall in one piece, but as soon as it was off, he was unable to lower the mirror slowly enough to the counter (since he was still standing to the one side), so a small corner broke as it hit the counter.
If we were to do that again, I think we would avoid the breakage if we just had a thick sponge on the counter to cushion the fall. ( i.e. I wouldn't recommend that someone stand there to catch the mirror - just in case.)