MAGNETIC SHIELD FOR MY ALLIES: Give Magneto a power action to attach the Magnetic Shield to another friendly character, removing it from anywhere else. That character can use Energy Shield/Defelection as long as it is attached. When that character takes damage from a range attack or Magneto is KO'd, remove the Magnetic Shield marker.

Can Magneto attach the Magnetic Shield to someone with a 2x2 base, even though he can't actually attach the plastic Shield piece because of base size?

The FX markers are, as the name indicates, just markers... they serve no *REAL* function, other than to remind you who has what.

If you don't have one on a character, for whatever reason (base too large, marker has been lost, etc), just mark the character some other way.