Although this topic should most likely properly be in the gunsmithing section, IMO what you're referring to is called the "length of pull", i.e. the distance from the center of the BP to the center of the cocked rifle's trigger.
Different manufacturers cut their buttstock to various different LOP's - why it's generally advisable to try before you buy.
What usually works for most folks is an LOP measurement that matches the distance from the inside of the shooter's angles elbow, to the center of the tip of their trigger finger.
Sooooooo, cradle any long gun in the crook of your arm (inside of the elbow), and extend that hand to see where the trigger finger falls on that particular gun.
For a proper fit, the finger should just fit on the trigger - if the finger doesn't quite reach, the stock's too long; if the finger reaches fwd/past the trigger, the stock's too short.
In whatever case, the LOP might need adjustment: shortening via cutting the rearmost end, or lengthening via the addition of a thick recoil pad or (in extreme cases) a wooden filler strip betwixt the present end of the buttstock & the BP.
LOP can also be lengthened by wearing a shooting shirt/jacket with a padded area on the front of the shoulder, which has the same effect as lengthening.