how to measure a western saddle

I have a western saddle recieved as part of a trade. How do I measure to determine the seat size and what size tree (bars?) it has (it seems fairly narrow)? Also how do you know what type of rigging? It has? I would like to be able to describe it reasonably accurately as I am probably going to sell it. I believe it is a pleasure saddle,no obvious brand name, black with a padded suede seat and too much silver for me to want to keep clean.
TIA from an english rider