Is it bad if you wear shoes that are not your size?

Not necessarily. Wearing too tight of shoes will most likely just cause blistering and general discomfort while walking. The blistering could become a problem if this is a habitual practice of wearing too tight of shoes and little medical care is administered to the wounds. On the other hand, too large of shoes will most likely just flop around, but may also cause blistering as they will continue to rub against places like the heel.