The C++ standard doesn't say anything, but in most of the platforms C++ use the single/double precision standard from IEEE, which define single precision as 4 bytes, and double precision as 8 bytes.
http://en.wikipedia.org/wiki/Single-precision_floating-point_format
http://en.wikipedia.org/wiki/Double-precision_floating-point_format
I'm not sure about the exceptions for these cases.
