The horsepower needed for your garage door opener is dependent on whether or not you want a long lasting opener verses replacing it every few cycles, or the size of the door..Ft garage door is the size that is suitable for you who have a oversized garage. Usually ft door used to accommodate two vehicles in the garage. There are a few sizes of doors that are considered standard..Double Garage Door Sizes. As it concerns double garage door sizes, the standard doors have measurements of feet, feet and feet..If you plan to replace your existing door, do not justume it must be some standard size. There are some standard garage door sizes, but there are also plenty of exceptions..