length is one thing, but also how much weight the trailer is rated for. and then how much the pulling vehicle can handle.
other words, weight is going to be on the top of the list.
www.tractorhouse.com and click over on construction equipment, then TLB (tractor loader backhoes) i can never remember correct address for tractorhouse sister site that handles construction machinery. going through used machinery and what is available should help ya get some idea of make / models out there. and might help ask for lengths and weights of machinery.
you might look in to a "excavator" or rather make that a mini excavator. or a skid steer with backhoe attachment.
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with above said what are you going to be doing with the machine? folks might be able to help point ya in a better direction if that is known.