Internet to the rescue!
With a tape measure, measure the inside length of the shoe. If that's too hard, as it sometimes is depending on the style of the shoe, measure the sole length instead.
Then go here:
http://www.i18nguy.com/l10n/shoes.html and scroll down.
He has a grid which will tell you which measurements correspond to which shoe sizes for women, men, US, other countries, etc. etc. etc. I've used it dozens of times.
www.fullservicebohemian.blogspot.com
