whats the smallest size, when you divide a fern from a rhizome, that will survive?

i was cleaning up the patio and saw a bunch of fronds coming up out of the dirt that spilled over onto the concrete. The dirt was only about an inch deep and they wouldn't have been able to grow to any signifcant size in that area. i dont know how they got there, but i dug some up and put them in pots.

most of the divisions are about 6 inches long, but I was wondering if I could divide them up even further.