The length at each end is set by the distance between the center roller and the two fixed rollers on the machine in question, plus a margin to start/stop the pipe.
FIRST and before ANYTHING ELSE find some one in your area or the fabrication area who already has the machining, the cranes and layout tables for 20 inch pipe, and the transportation to get the pipe to and from his shop to where ever you need the bent pipe. That company (for a twenty inch pipe capacity) has invested many tens of thousands in a machine not often used. Expect to pay a lot.
THEN, ask that company how far they can bend successfully and reliably 9or with what number of failures to successes for each bend diameter. Far, far more than steel welding, pipe bending is an art that doesn't always work. You will find the roller and foreman far more important than the company salesman. Do NOT believe just thumbrules - skilled rollers will be able to break thumbs and thumbrules.
Specify a bad setup, you WILL pay for the many failures between each successful bend. Set up a good, easy, well-practiced bend on a table and machine capable of that work, and you will get good bends all day every day.