The following chart gives you the industry standard for PTO drive line horsepower ratings. Please keep in mind these are rated horsepower at the PTO, not engine horsepower. Many times the difference between engine horsepower and PTO horsepower can vary as much as 10%. These values give the maximum recommended horsepower that can be transferred through the drive live from the tractor to the implement.
If you are not sure how how far you can safely extend a driveline, the following chart give the recommended maximum extension available on each series of shaft with a minimum collapsed lenth listed. In most cases, it is recommended that the driveline does not extend over 1/3 of its' collapsed length. This means the driveline should should still overlap 2/3 for safe operation at all speeds.