This is a snap on bandana but if you'd like to tie before you snap I'd suggest sizing up. Please also consider the fluff of your pet when choosing a size. If a collar snuggly fits your dog at 17", I would suggest size Medium instead of Small because 17" is as far as the Small will go and you'll want a little wiggle room.
Sizes: XS - up to 13" S - up to 17" M - up to 21" L - up to 25"