I am going to buy Baxter a faux leather jacket but am not sure of which size to get. The size small which I usually get him says it fits 8" to 12" and the meduium fits 14" to 18". I measured Baxter from neck to tail and he is 12". I'm not sure if he will out grow this real soon or if I should go up one size. His weight remains consistent but I haven't measured length in awhile. What do you guys suggest?