Before a 'C' program can run on any processor, it has to be compiled using a compiler for that specific precessor, Therefore you have to know the processor - and, therefore, its size - before you can even compile the program.

Compilers usually provide some predefined macros that can be used to identify the particular compiler; eg,

#if defined _MSC_VER
// This is Microsoft 'C'

#elif defined __GNUC__
// This is GCC

#elif defined __BORLANDC__
// This is Borland C

#elif defined _CC51 
// This is the Tasking 8051 Compiler

#elif defined __C51__ 
// This is the Keil C51 Compiler 

#else
#error Unsupported Compiler!
#endif

See: http://www.8052.com/forum/read.phtml?id=136086