Ha ha,,,,to easy huh? Yeah right!!!!
You gonna HAVE to do some research, there is no free lunch! But yeah, I'd start with the deminsions of your existing shock. And there IS a formula as to figure out HOW and WHAT to measure with your existing shock to get an exact replacement.
Exact replacement? Nah, to easy! Do the math thing.
Check it out here:
http://www.koni.com/_cars/frame.html
Ernie