Finding a complex structure size

I have tried variations using sizeof() with unreliable results. This method works, but is it bogus? Can it be improved? I am not worried about it being portable.

// -- Unit Variables --
struct {
  // Complex Structure
  // Lots of nested arrays,
  // integer values, etc.
} message;

char replyBuffer[20];	// Input Buffer


// Return size of message structure
unsigned int getMessageSize(void) {
   int i, *p1, *p2;

   p1 = (int *)&message;      // Create pointer to Message Struct
   p2 = (int *)&replyBuffer;  // Create pointer to replyBuffer
   i = p2-p1;		      // Calculate message structure size
   return(i);                 // Does this really work?
}