I think 15' is probably bigger than a lot of people realize. Looking at that picture, it seems they are using 14" square and 28" rectangle tiles (common size). Extrapolating those sizes (excuse my poor line skills), we can see that the snake fits in what, 12 tiles? That's 14 feet.
View attachment 67730
Yeah, looking at that and other pictures on the internets, it fits better as a Huge.