Answer: Extremely stubborn and suspicious
When Mary reads out the measuring tape's estimation of him, he insists 'I am not!'. Michael's suspicion (that Mary Poppins is clearly a witch) causes him to go through her carpet bag looking for evidence. He finds nothing, of course, although she subsequently extracts a plant, a standing lamp and a singing mirror, amongst other things. 'Close your mouth, Michael, we are not a codfish.'