Understanding VirtualSize and SizeOfRawData in PE Section header

[Stephen Torri]

I am trying to understand how the VirtualSize and SizeOfRawData are used
in the PE Section Header when Windows loads a executable or DLL into
memory. So far here is what I understand. If the VirtualSize is greater
or equal to the SizeOfRawData then the loader reads N bytes of section
data with N = SizeOfRawData. Yet if the SizeOfRawData is greater than
VirtualSize then it was rounded up to the nearest segment so the loader
reads N bytes of section data with N = VirtualSize. Is this a correct
assessment of how these two variables are used? My problem is how to
determine the real length of data to be read from the file.

Stephen