While standing, have someone trace an outline of your feet (both of them, as one may be larger than the other). I've heard some onlines will use this to size your feet.
Or, find a place allows easy returns (ie, prepaid return labels) and order your (man) size + 1 (for American sizes) (or round up to the next whole number).
But shoes are like other clothing items...just because a size 11 fits in one shoe style, doesn't mean another style will also need to be size 11...