UGG boots generally run a half a size to one size bigger, according to ShoeSurfing.com. This applies especially to Classic Tall and Classic Short, Ultra Tall and Ultra Short, Sundance Short, Sundance II and the Adirondack Collection. The Fluff Collection, however, runs true to size.
UGG Australia recommends that buyers select a size smaller than their regular size when buying any of the styles that run big. Half-sizes, on the other hand, should be rounded down. UGG boots often feel a little snug when they are new, but with time, the fleece interior molds to the wearer’s feet, creating a more comfortable fit. UGG Australia’s website has a size chart for customers who are unsure of their shoe size.