G4UserLimits class uses DBL_MAX as default value, but this is never used in our code.
Further details
These slides briefly explain the purpose of the epsilon max parameter: The MaximumEpsilonStep parameter is the relative accuracy for the endpoint of 'ordinary' integration steps, those which do not intersect a volume boundary. Limits estimated error |Delta x| of endpoint of each physics step (of length len): |Delta x| < Epsilon * len
The other set of slides mentioned above contain the following default units, which might eventually come in handy:
Length: mm
Time: ns
Energy: MeV
Angle: Radian
Temperature: Kelvin
All units are defined at source/externals/clhep/include/CLHEP/Units/SystemOfUnits.h (which also specifies the basic (default) units)