It is important that shoes fit your feet correctly. The most frequent mistake we make is buying shoes that are too narrow or short for our feet. This squashes the toes, puts excessive pressure on the feet and stops the foot from functioning properly. Shoes that are too big allow the foot to slide around and don't give adequate support.
The perfectly fitting shoe should be wide enough to fit your foot without feeling squashed or pressure anywhere. There should be at least your thumbnail width between the end of your longest toe and the toe of the shoe. You should be able to wiggle your toes freely and comfortably. Your heel should not slip up and down.