[deroko]

int main(){
        int size;
        __asm{
                mov eax, offset mylabel
                sub eax, offset main
                mov size, eax
        }
        printf("0x%.08X\n", size);

__asm{
        mylabel:
        }
        return 0;
}

smthing like this? If I remember correctly msvc will compile this without a problem even without asm code, so just put label at the end of procedure and calculate it's size... that should do the trick.