I believe other folks have mentioned that when the radials are buried,
no specific length is required for the antenna to be resonant. The more
radials you use and the longer they are, the lower the loss. There's a
point of diminishing returns for both length and number, and it turns
out that if you have only a few radials, making them very long doesn't
help much.

Elevated radials normally have to be close to a quarter wavelength long
for resonance unless they're very close to the ground in which case they
need to be somewhat shorter. You can use the same techniques to shorten
elevated radials, though, as you do a vertical radiating element. You
can add a loading inductance at the feedpoint or farther along each
radial, you can use a capacitive "hat" at the end, or some combination
of the two. And just as happens with loaded verticals, the result will
be narrower bandwidth, lower feedpoint resistance at resonance, and
potentially higher loss.

Roy Lewallen, W7EL