Where have you been hiding?...
I try to take the old spring and match it up whenever possible.
If you dont gots...then go to the local hardware or garage door supplier with a picture of the door...the size and type of material.....
Just kidding, take the other spring with you and get 2 new ones...same size.
Good luck and be careful with the removal of the other spring...and the door needs something to keep it up while you get your springs or it will come crashin down.
You should also have a wire that runs through the spring to keep it in place when it does break again ...someday.
All that for a spring...whwew