It seems that a 50 degree angle is standard for an A-Frame style trailer coupler bracket. If you plan to use one of these, then buy it before you build this.
My trailer is 4' wide (frame only) and the tongue is about 5' long.
Buy your trailer tongue box first too. You'll want to check the trailgate on your tow rig, to make sure everything is going to play well together.
Check this picture: