Two options, replace with shocks of the same dimension that you are currently running, if they work for you now, or flex your truck out to it's max and take compressed and extended measurements from the upper and lower shock mounts both front and rear. Take the difference of those two numbers and that's the amount of shock travel you need. Double check the shock you're looking at with the compressed and extended numbers and pick one that will work for your truck.
I recommend Bilstein 5125's or Poly Performance's BBCS.
for reference I run Billy 5125's, 14" up front and 12" in rear BUT....
as others have said, variables such as mounting config and spring choice make it important to measure for your application.
with that being said, as far as bilsteins go, they offer the 5125's in three different sizes (10, 12, and 14") that could apply to your SO truck so you don't have to get out your graphing calculator or anything like that.
If what you have works when extended and compressed use those.
I personally use off the shelf OE shocks from other vehicles so they are easy to get my hands on and cheaper. Measuring extension and compression and then deciding on mounting locations then becomes the harder part of things, but its all easy.