Before buying cowboy boots consider the country that they are made.
Uninted states and Mexico Use the metric boot sizes for example 7.5D (means 7 1/2 Us normal fit) in australia means size 7 as australian sizes are 1/2 siz smaller than US.
Mexico use both sizing 43 or 7.5D
Italy use Europe sizing 43, 7 1/2, also 1/2 size larger than Aust.
English Uk sizes are same as Australian sizes.
For example if I wanted to buy a pair of boots from US size 8 Australian I would buy a pair from Us size 8.5.