Here is a decent link to a mecate set up on a Bosal. Bosal
As said it is ussually 22-24 ft. Long. Made of Horse hair or nylon rope or yacht line. Generally speaking you want the macate to be the same diamiter as the bosal you are using. If you do get horse hair mecates try to get Mecates made from mane hair. It is alot softer than the tail hair Mecates but the are alittle more pricey.
Here's a drawing of how one goes onto a snaffle with slobber straps. NRS - National Ropers Supply - Western wear, horse tack, horse trailers, team roping, cowboy boots & hats
I use both Horse hair and nylon. I prefer the horse hair for the feel they give the horse but the price makes it tough to own a bunch of them so I do use the nylon ones to.
About the Bosal. Don't get a really cheap one. They are one of those things that you get what you pay for. The more plaits of rawhide the have the smoother the surface. Most are around 12 palit with a 16 plait nose band. They are pricey as well but a good one willl last if taken care of. Also make sure it has a rawhide core or at the very least a rope core. The wire core bosals are a no no. They have very little "feel " and can be pretty rough on the horses nose.