Case length is important. If it is too long for the chamber, the mouth of the case will be forced into the bullet, and peak chamber significantly elevated. This is a safety and accuracy issue. If you are reloading a standard cartridge (not a wildcat, or chambered with a custom reamer) you can find maximum, and trim to case lengths in a reloading manual.
There are several methods for measuring a rifle's chamber to determine bullet seating. Hornady and Sinclair sell gauges for this purpose, as well as gauges for measuring you refer to as shoulder length.
What you call shoulder length is important. I fit is too long, a rifle's bolt will be difficult, if not impossible t close. If it is too short brass life will be drastically reduced, and ignition reliability may be affected.
When starting reloading, your first purchase should be a reloading manual, and the one piece of equipment that you must have is a powder scale. I would also suggest that if you have a friend that reloads that you consult with him.
Please feel free to ask as may questions as you like, but in the future, basic reloading questions would be better located in almost any forum except this one, which is designated for matters directly pertaining to shooting in actual matches. The Factory/ Hybrid forum would be a good one to use for this sort of question.
I remember my own start in reloading. It was with a Lee Loader. I used a plastic headed hammer to apply the force needed to decap, size, prime and seat bullets. Powder was measured with a dipper.
Today, I have almost every tool imaginable, and I am not sure that I have any more fun than when my hobby was a lot simpler, and I knew a lot less.
Good luck with your new endeavor. As with anything relating to shooting, there are safety issues to consider. If you don't really know the proper procedure, don't proceed without determining what it is, from a good source. In the unlikely event that something was to go wrong, having some eye protection on is always a good idea.
