Unless you're using something really oddball you should be able to pick the specs out of the projectile library.
In the top line(of the projectile section) is the magnifying glass - click on that and find your bullet. It should provide you with all the values you need. These are usually spot on, though as with everything it never hurts to do a quick sanity check!
From memory the dimensions it requires are the diameter of the projectile at the bottom end of the boattail, and the length of the boattail i.e. from the base to where the projectile becomes full diameter.
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