Using direction valve of Simhydraulics

I am using Matlab 2008 and using the module of Simhydarulic to build a electrohydraulic circuit working and controlled by PID controller. This controller control servo 4/3 valve. I put the transfer function of the servo valve but for the 4/3 direction control valve i dont't have maximum area and maximum opening. Actually the required response from the simulation get only when i am using max area 10 mm and max. opening 10 mm. If i want to change Area to be 100 mm (which is right value) i get very bad response (Highly oscillation). but if use area 10 mm i got the actual response as real plant. By the way if i use max. are 10 mm2 this impossible coz. that is mean the spool diameter is .33 mm which is not practice value. Any one help me to solve this problem?