without giving you the answer, try looking in Michael Niu, maybe Raymer, or Torenbeek ...
the answer does depend a little on what you mean by "hydraulic ram" ... actuator ?
you could look at existing designs (try specialist undercarriage companies like Cleveland, Goodrich, Dowty)
you could think about the environment (hydraulic fluid), type of airplane (space constraints to would tend to high strength materials), cost issues (military, large commerical, small commercial). something real is going to be experience of the airframe company and the specialist undercarriage company (they may have design guidelines driven by experience or "urban legend").
Check again on the type of aircraft so as to know strength required, drags expected, the size of the ram to come up with.
Material availability is a great concern too.
For most Boeing aircrafts, the undercarriage itself is made of steel (structural) and so are the actuators. Seals may be made of varying materials dipending on the fluid you have chosen.
That reminds me, you need to consider Maintenability.This includes; repair, replacement, lubrication, removal, modifications...and a host of such...
