Measure the length, the diameter of the cylinder and the diameter of the rod. There is some wall thickness of the cylinder...under 3/16 of an inch. The diameter will be in even inches or half inches generaly so use the closest measurement under the cylinder diameter.
Now it is math. The area of the cylinder (pi * D squared) / 4 times the length to get the volume, minus the area of the rod (same formula) times the length is the conservative way for pump pressure. The larger volume is without taking the rod volume out and for pump capacity is what you should use remembering the force (pressure in psi times the cylinder area) will be greater in that direction.
Ted