We played it as the rogue would destroy the box in returning to normal size, but they take damage equal to the boxes health before it explodes.

My favorite situations were her getting trapped in a floor safe for an entire fight because the bandit immediately locked up the pretty stone, her erupting out of the pocket of a maid being interrogated for stealing from the Lord, and her exploding from a coin purse on a Duke nearly a county over because the he picked her up while leaving the bank instead of while entering.