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Ignoring the permissions and focussing on practicalities, the most stable method of towing a dinghy behind is with a vee shaped arrangement, from towboat one side back to the dinghy then back to towboat other side....stops the dinghy sheering about from side to side.
I'd start with about 20ft of rope but from a blind guess I'd say have 30ft....you can always shorten but if it's too short you'll get bumped from behind every time you take power off and may also end up with a dinghy full of propwash.
Just in case you're tempted I'd also advise against having anyone in the dinghy whilst under tow...especially youngsters....seen it done lots but think through the 'what ifs' first....