The cables on any garage door are gauged on two things. The weight of the door and the length of cable needed. The weight of the door will determine the gauge or thickness of the cable. And the length of the cable is determined by the type of spring system your garage door has.
The two basic spring systems are torsion and extension springs. The torsion spring system uses a lot shorter cable just about a foot longer than the height of your garage door. Extension spring systems use about four times that length of cable. This is because extension spring systems work on a pulley system and the cable has to run from the bottom of the door all the way up and to the back of the door and back to the front of the door where it is tied off. Also there is or should be a safety cable that runs thru the center of the spring that is there so when the spring breaks the cable keeps the spring from flying thru the garage and causing any damage or injuries. So on a extension cable system you would need about 30 ft. of cable for each side so about 60 ft. total.
Cable loop at the bottom
Cables need to have a loop at the bottom to attach to the door and a cable ferrule (A ferrules an object made of metal used for the joining or looping a cable) at the other end that hooks into a slot on the cable drums. Extension cables just need a loop at one end because the other end is just tied off into a not at the other end. If your garage door has a torsion spring system and the spring assembly is at the rear of the door than you have what they call rear torsion and you would need about 16 ft. of cable for each side of door for this type of set up.
As far as the strength of the cable, the heavier the door the thicker the cable. Doors constructed of wood will use a lot thicker cable than a metal door. But having thicker cable than you need is never a problem.