What is your background? Are you a mechanical engineer?
So given an oil flow rate, you must then select a cylinder diameter to generate the force required. This is because flow rate, diameter, and pressure determine force.
D = sqrt(4*Q/(pi*V))
# D - pipe diameter
# Q - volumetric flow rate (m^3/min)
# V - velocity (the speed that you want the piston to move)
oil is incompressible
Now, using static force analysis
, position the piston so that it applies force just where you want it.
(hoping that made sense, I simplified it as much as I could)