What size torsion spring does my garage door need?
Every door based on weight has has its own very specific spring or springs.  No guessing.  You can take your complete old spring to an overhead door company and they will get you the correct replacement. While we are on the subject, the changing of an overhead door spring is really not a job for the home DIY guy. This is dangerous.  If you have a high level mechanical aptitude, maybe; but watch the videos on YOU Tube first.  And be careful.