ָУ меня есть Draft версия стандарта от 1 сентября 1998 года. С тех пор, я не думаю, что много изменилось в этом отношении:
Параграф 3.9.1 Fundamental types, абзац 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; the other signed integer types are provided to meet special needs.