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))

where

# D - pipe diameter

# Q - volumetric flow rate (m^3/min)

# V - velocity (the speed that you want the piston to move)

assumptions:

oil is incompressible

room temperature

laminar flow

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)