IT Staff Size

I am the IT mgr for a small shop that supports 150 users in 14 locations throughout the midwest and also supports VoIP for 3 of the larger locations. I am looking for some outside criteria for determining staff size. I realize there are a large number of variables to consider, I am just looking for some surveys or models comparing size based on whatever criteria, number of users, locations, overall company revenue, etc. ? Does anyone know where I can find this sort of info ?