I can't measure them!
Easy, just remove the elastomers or any remains of, extend the fork and put something in there to measure the depth - something like a bit of copper pipe? - mark it, measure it - from what I can remember the elastomer stack or spring was always a bit longer/higher than the top of the stanchion, 5-10mm maybe?
If you get springs remember to make sure they will fit into the top cap part? If you get elastomers and need to cut down and have those black plastic spacers inbetween small lenghths, remember to account for these too?
Then one day you find, 10 years have got behind you ....