We have a parent with a 6x12 that retails for $4,000 and required a good size truck to pull. For the fall classic (a southern California off season event) we used it to transport our 08 robot, a prototype crab, several toolchest and rolling carts, a large table and still had plenty of his heating and air conditioning equipment (the purpose of the trailer usually).
So for referance, I'd say something smaller than 6x12 will do fine
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