Have you measured the amount of Z movement? The weight of the head and the amount of movement are the two things to figure out.
Then, are you going to make brackets so that you can mount the pneumatic cylinder(s) in a straight line up and down, or are they going to be angled. If angled, then you should figure out what the angle is at the top and bottom of the stroke which depends on the mounting position. The upwards force will be the cosine of the mounting angle x the force from the cylinder. So at zero degrees (vertical), it's 100%. At 30 degrees, it's roughly 87%.
Once you roughly figure out how you are going to mount them, then you can pick a stroke for the cylinders from the different models that are available, then you will know the exact dimensions to complete the design.
I can let you know more when I finish ordering my parts. I've never done this before either. Obviously you've read my thread.