On most current 64-bit systems, a long int is 64 bits and an int is 32 bits. However, as AD said, look in /usr/include/limits.h (Unix/Linux), or wherever limits.h is found on your system. You can also use the sizeof(int) and sizeof(long int) in your code to determine what they are at run time, just in case your code can run on different architectures. Example:
Note that this is vanilla C code. It works just as well in C++, and you could use cout << "sizeof(int) == " << dec << sizeof(int) << ", sizeof(long int) == " << sizeof(long int) << endl; just as easily.