I would recommend removing the stud, take it to any trailer supply store and have them measure the current stud. Buy a suitable replacement AND A SPARE !!!
The reason I suggest this route is because various axle manufacturers use different size studs, sometimes on the same axle assemblies. I ran into this in North Dakota several years ago. What Lippert "said was supposed to fit" was not what "actually fits".
I've found that the easiest way to remove the damaged stud is to use a heavy C clamp, back the stud with an appropriate sized socket (so the stud can be pushed inside the socket as it is pushed out of the backer plate) and position the C clamp, then tighten it to push the lug out of the backer plate. Installing the new stud is the reverse of the same process with the socket on the inside of the backer plate.
As for length of the new stud. Measure the current ones, and realize that there is typically "at least 1/2" of threads remaining when the "open face lug nuts" are installed and if there is 3/4" or even more, as long as the stud doesn't interfere with the hubcap, it functionally doesn't impact the function of the wheel/axle assembly. If you have "acorn lug nuts" this doesn't apply.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS