It will probably have a code on the side of the cylinder in the form of a part number, quote that to the manufacturer and they will help you.

Or work it out urself?..I have an SMC branded ram with me at the momment, On the side it sais C92SB32-320 The end of the code tells me that its 32mm bore and 320mm stroke. Once you have the bore of the ram, ordering a clevis should be no problem.