Let's start out with what you know and what you got!
What kind do you currently have?
Measure from center of mounting hole to center of mounting hole.
Stroke is the amount of movement the shock covers, i.e. how much travel does the shock have. If it's an air shock, you can release all the air and compress it all the way, then measure the distance, then pull the piston all the way out and measure again. the difference between the two is your stroke.
The best thing to do once you have this information is to contact the manufacturer of your existing shock and verify to determine what exactly their measurements are.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger