What you're asking about is defined (or not specifically defined) in the C++ standards. Size of an int should be the natural size of the achitecture - many of us grew up with 2-byte ints. The standards don't prescribe specifics, it's more on the order of any larger data type must be at least as large as that that comes before it. Thus, today you see that int and long int are generally the same size.
3.9.1 Fundamental types
1 Objects declared as characters (char) shall be large enough to store any member of the implementation’s basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (3.9); that is, they have the same object representation. For character types, all bits of the object representation participate in the value representation. For unsigned character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined.
2 There are four signed integer types: “signed char”, “short int”, “int”, and “long int.” In this list, each type provides at least as much storage as those preceding it in the list. Plain ints have the natural size suggested by the architecture of the execution environment(39) ; the other signed integer types are provided to meet special needs.
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39) that is, large enough to contain any value in the range of INT_MIN and INT_MAX, as defined in the header <climits>.