My Garage door has two torsion springs for a 7'X 16' door. The springs are two different sizes. How do I know which size to replace it with?
Yes, the spring sizes are based upon the weight of the door. And sometimes a particular door can require two different size of springs. Door spring sizing and engineering is a very sophisticated science. Your best bet is to have a pro come and replace the spring for you. (replacing an overhead door spring is a dangerous proposition.)
⚠️Warning — Adjusting garage door springs can be extremely dangerous. The spring can move suddenly, causing serious injuries. Before you touch your own springs, learn how to release the tension safely.
If you feel like you are up to the task then first relieve the tension on the unbroken spring. Remove the broken spring. Take it to a door shop and replace it like for like. Install new spring and tension to specs. (The number of turns is based upon the height of the door.) (Tensioning with two different sized springs on one door is particularly tricky.)