FOR BOOTS AND SHOES:

This chart is a good rule of thumb.  Most dog shoes and boots are fit to these standard measurements:

XXX-Small fits up to 1 1/2" XX-Small fits 2 1/4" X-Small fits 2 3/4" Small fits 3 1/4"

Medium fits 3 3/4" Large fits 4 1/4" X-Large fits 4 3/4"

CAPS, HATS, AND VISERS: 

Most Hats and Caps are made to fit Most Dog Sizes:  with velcro or snaps the large, medium and small descriptions are adequate:

BUT THERE ARE ALWAYS EXCEPTIONS TO THE RULES:

Here are some measurements that will help determine the sizes for your dog.

It really depends on the style of head wear you are getting.

FOR CAPS:

FOR HATS (Like Cowboy Hats and other Fancy types)

FOR VISORS: