What screwdriver size is needed for MacBook Pro mid 2012 back cover?
According to iFixit's teardown the Philips #00 should be used for the "lower case". Is the lower case a synonym for the "back cover"?
The screws are pentalobes and should not be mixed by Philips or torx. If you usa a torx screwdriver you will damage the screws and in worst case you'll make them flat and you will not be able to unscrew them.
More info on pentalobes: http://en.wikipedia.org/wiki/Pentalobe_screw