My last draft ran away from me, so I'm going for something simple with no capacity for power creep
A safe SCP which is a collection of windows. On one side of each window is a plus and on the other side a minus. When something passes through the side with a plus it grows by ~15% and if it goes through the side with a minus it shrinks by the same amount.
The only containment procedures would be keep them shut and stick them in a storage locker.
To avoid the SCP from making every air molecule a mile wide in a day, I'm thinking of restricting the effects to above the molecular. The only detail I'm not sure of is if it should change the density of the objects it effects or add and remove mass.
Is this a good idea, or does it need more than a dumb pun to work?