const std::string::size_type cols = greeting.size() + pad * 2 + 2;
int is supposed to work! it holds numbers!!!
A short holds numbers too. As does a signed char.
But none of those types are guaranteed to be large enough to represent the sizes of any strings.
For a simple example of why this is necessary, consider 64-bit platforms. An int is typically still 32 bit on those, but you have far more than 2^32 bytes of memory.
So if a (signed) int was used, you'd be unable to create strings larger than 2^31 characters. size_type will be a 64-bit value on those platforms however, so it can represent larger strings without a problem.
The example that you've given,
is from Accelerated C++ by Koenig. He also states the reason for his choice right after this, namely:
There's no point in using it in application code, a