well if you leave the settings at zero mental Ray will automaticaly try to set the values for you. But, if you wish to change for better accuracy just take 10% of the over all scene unit size and enter that into the max radius. For minimum enter 10% of the Max Radius. This is just a general rule.

e.g. if your scene size is 100 units wide enter 10 for max radius. For min enter 1 which is 10 % of 10.