The largest pipe clamps I have have a jaw to jaw distance of approximately TEN FEET.
This idea didn't originate with me, I read about this years ago in something like Popular Mechanics or Mechanix Illustrated.
Use common pipe clamp hardware you can buy most anywhere. Illustations of the basics have already been posted in this thread. Buy 2 10-foot lengths of rigid threaded conduit. These have a female socket welded on one end, and a male pipe thread on the other end. Rigid metal conduit was designed to be simply screwed together to make very long runs, I supposed you could link 2 (or more) 10-foot lengths together for a jaw-jaw distance greatly exceeding 10 feet.
Anyway, the male threads on this type of conduit are compatible with the threads on a common pipe thread element, just slip the sliding part on first, then screw on the adjustable part, and -
voila you have a REALLY BIG pipe clamp. You can easily unscrew the clamping elements to store the conduit when not in use. When I bought the rigid conduit lengths, they were much cheaper than threading a long length of pipe would have been.