That isn't necessarily true. I have brochure here from back when the 4X10 series was introduced. They show the tiny 4010 be compatible with an 8' box blade. Nobody in their right mind would believe that, yet it's right there on page 25. Point is, just because JD info might say it somewhere doesn't necessarily make it true.
The 1 series may have enough power for a 5' BB, but traction/weight is much more of a limiting factor than lift capability or HP. Use a 5' wisely and you won't have too many problems. Just don't try to make a big tractor out of a little one, and load it to the max.