You can use fixed width (or guaranteed-at-least-this-many-bits) types in C as of the 1999 standard, see e.g. Wikipedia or any decent C description, defined in the inttypes.h
C header (called cinttypes
in C++), also in stdint.h
(C) or cstdint
(C++).
You certainly can check for each computation what the values of the values could be, and limit the variables acordingly. But unless you are seriously strapped for space, I'd just forget about this. In many cases using "just large enough" data types wastes space (and computation time) by having to cast small values to the natural widths for computation, and then cast back. Beware of premature optimization, and even more of optimizing the wrong code (measure if the performance is enough, and if not, where modifications are worthwhile, before digging in to make code "better").