I would look at your hoses and see if there is any markings about pressure marked on them, they should be rated higher than your working system pressures. as far as the cylinders go I would take them to a hydro shop and ask for a estimate on a rebuild, they can measure them and get approperate seal kits usally.
you can not really go by outside dimensions except for the rod, all the seals will most likely be metric if you are having it repaired. you have the needed dimensions just to buy new cylinders ,except the hole size on each end and make sure the hoses and the cylinder threads are the same.