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I just can try to:
The "max radius" is of course the maximum blurring radius applied to the image in pixel width of the 1:1 scaled image.
So if you downscaled the image, you have to use larger sizes.

The "object size" parameter influences the radius along the Z distance, from the nearest part of the object to the
farest.  Might be not physically correct, but a viewpoint isnt it neither.

So a low object size value should tend to a more microscopic look, a bigger one should tend to a view of a larger object.
Especially when using a larger value the maximum radius could be only in the nearest distance, so you need larger
"max radius" values to archive some blurring.

All i can say is that it surely does not depend on the fractal type, the DoF effect makes no assumption on that, just relays
on the Zbuffer values and the already calculated colors.
As said, higher values needs often greater radius values, and it depends on where the focus lies.
If the focus is not on very near parts, the blurring should more effects the nearer parts and left the farer parts as is.

But if 0.5 is ok for you, than go with it.