Length of radials on the ground is not critical
Actually it does not even really matter. You can use any length you can fit in.
You are correct. From my own experience I use a combination of 35 foot, 50 foot, and 100 radials on the ground for my 160 meter inverted L. I now have a total of 50. As I added radials, regardless of length, I could see the feedpoint Resistance drop significantly to the point where it would stabilize at about 40 radials.