Probably one of the best-known symbols for good luck next to the four leaf clover, the horseshoe is a mainstay of house magic. The iron in the shoes is said to repel evil (particularly the Fae, notorious for mischief like souring milk and hiding your keys).
Affix to your door or window with the points facing up, otherwise your luck may run out the opening at the bottom! There are those who believe you should nail it with the points down so that the luck and blessings will flow downward upon the people who pass underneath. We're on team "points up" but a good part of magic is going with what feels correct for you so follow your heart.
Size varies, these are on the small side (think Shetland pony rather than Clydesdale) and are roughly 3"x4"