Mirror size
Step one is to determine what size custom towing mirror you need in order to be safe and legal. While each state has different laws and regulations, all of them agree on certain basics that are dictated by the width of the trailer you’re towing and its length.
Trailer width
Whatever the width of your trailer, the side mirrors need to extend out far enough so that when they’re properly adjusted, the driver can see the entire length of each side of the trailer. To ensure you can see the side of the trailer, each side mirror needs to extend past the side of the trailer. For example, if the trailer you’re towing is eight feet wide, the distance between the outside edge of the two side mirrors needs to exceed eight feet.
Trailer length
The longer the trailer you’re pulling, the more difficult it will be to see anything directly behind you. Ideally, you should be able to see anything within one car length of the trailer’s rear bumper. Even the best towing mirrors sometimes fall short of that ideal, but it’s the goal you should aim for. The longer the trailer, the further out the side mirrors will have to extend in order to give you that view behind.