Sizing is most important with ugg boots and other sheepskin footwear as the natural hide tends to stretch over time. The wool also compacts as it moulds to the shape of your foot, so for both these reasons we strongly recommend to select the size one below what you would normally wear in other shoes. This is to avoid disappointment as the boots will end up too big if you choose your regular size.
There are some sizes specific to particular boot styles. The sizes for these boots are displayed on the page for those products. In Australia our shoe sizes follow the UK convention so a table mapping these sizes to the US and EU measurements is below.
Gee Sheepskin Ugg Boot & Slipper Size Chart