I think this can vary slightly from time to time depending on conditions the cigar has been in (as far as heat and humidity). That can change the length a little, but if its way too big or small and they just don't make them in that size then thats a red light right there.
http://www.cigarnexus.com/counsel/counterfeit/ heres a site with some good info. Cigar Aficionado also has pages on their site with a bunch of fake bands to show you what to look for.
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