This pattern can be tied in just about any size, but I've mainly seen it in size 12 & up - mostly sizes 12 & 10. tying it on a hook too small would make it difficult to tie.
This is an old pattern, first tied around the turn of the 20th century circa 1910. It get's it's name from the prairie squirrels in the western USA. These particular squirrels were named after the posts cowboys would carry to tie their horses to when in the open prairie & there were no trees handy to secure their horses. The way the squirrels stood up on their hind legs & their white coloration led the cowboys to call them "picket pin squirrels". The fly, who's wing is it's most prominent feature & made from the hair from a squirrel's tail, derives it's name from those critters.
Below is a good video showing one way to tie this fly. There are many different methods you can use to make them up & you can even get creative substituting different materials. So give the Picket Pin a try the next time you're fishing below the surface. You might just be surprised at how well the trout like this pattern.