If you look in the headers you will find
#if __POCC_TARGET__ == 3
typedef unsigned long long size_t;
#else
typedef unsigned int size_t;
In the help file is this.
__POCC_TARGET__
The current target processor specified through the /T option. Defined as the integer constant 1 for X86, or 3 for AMD64. [5.0]
So when compiling 64bit stuff size_t will be unsigned long long.
John