Who doesn't love the appeal of JFK Jr. This little classic Naval Officer Style Coat has a Patriotic air. Great for formal Occasions. --CLICK HERE for more information on determining size for this delightful outfit.
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For sizing, you should go by the size your pet is accustomed to and check the specific chart referenced above, but you may find the following a helpful guide for a quick assessment ( inches refer roughly to length measurements):

X-Small: 7" (2.5-5 lb) Small: 9" (5-8 lb) Med: 10" (8-12 lb) Large: 12" (13-20 lb) X-Lrg: 13" (20-31 lb)