Standard sizes are starting at 8' wide and going to 20' wide. Some door series have sizes smaller than that. Standard heights are 7' and 8' tall. Odd-heights and odd-widths are also available and vary by individual door series.
Because your garage door has so many parts that can wear out and break over time it is recommended that you look over you door every 4 to 6 months. An annual visit from a trained technician is also recommended so that they can check your springs to make sure they are in balance. NEVER EVER TRY ADJUSTING THE SPRINGS BY YOUR SELF AS YOU CAN BE HURT OR EVEN KILLED.
This is if one spring is obviously broken, the other spring is seeing wear as well and should be changed at the same time. They were most likely installed at the same time.