Roll cast length is directly tied to rod length. It is the length of line that hangs between the rod tip and the water that power is applied to, the more there is, the more energy can be applied to the line. This length equates to the length of the backcast on an overhead cast. The more line a caster can carry, the longer the cast, all else being equal.
Softer rods are usually an advantage also. The more a rod bends during a cast the faster the tip will be going when the loop forms, again, all else being equal. In a roll cast there is relatively little line to make the rod bend, a slower rod will bend more making the cast easier, and usually longer. Spey casters certainly know this, they use the longest rods around, and most are slow compared to single hand rods.

Of course, technique is more important than either rod length or stiffness. A good caster with a stiff 7 footer will probably roll cast farther than a beginner with a soft 9 footer.
Bruce