Honestly, the easiest way is to get the measurements of your axle travel and then just do the math. On the shock you have pictured, the cover is the piston, and the length of the piston is the total travel the shock can have. If you measure the difference between full sag of the axle and full compression (does it have a bump stop?), find a shock with a piston that length. In all likelihood, you can just take the length of the one you have and add the amount of lift you are getting with the new springs to get a pretty close new size. Most shocks either have the compression length in the part number or at least on the spec sheet.
Hope that helps.
Don
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