About the length of your trailer tongue. As you will have a single post tongue. I would make it 1/2 the width of your tow vehicle plus about 2 inches.
Then you can do this.
Has come in handy a few times when I have had to turn around out in the rough. And it keeps it as close to the TV as possible to keep in the wind shadow. Do not calculate the distance the hitch ball from bumper.
Randy