After you safely hitch your trailer to your vehicle, it's time to evaluate if you need side towing mirrors. As mentioned before, if you cannot see the entire length of the trailer behind you, you should invest in towing mirrors. Many times, if your trailer is wider or taller than the vehicle hauling it, the trailer will block your line of sight and could be hazardous to operate.
The size of your trailer will determine the added mirror height and the length needed. Then, think about how often you tow in order to decide whether a permanent or a portable towing mirror would better serve your needs.