Header with memory size definitions

As other answers pointed out, there is not. A nice solution in C++11 is to use user-defined literals:

constexpr std::size_t operator""_kB(unsigned long long v) {
  return 1024u * v;
}

std::size_t some_size = 15_kB;

No, there are no such standard definitions. Probably because the added value would be very small.

You often see things like:

#define KB(x)   ((size_t) (x) << 10)
#define MB(x)   ((size_t) (x) << 20)

This uses left-shifting to express the operation x * 2¹⁰ which is the same as x * 1,024, and the same for 2²⁰ which is 1,024 * 1,024 i.e. 1,048,576. This "exploits" the fact the classic definitions of kilobyte, megabyte and so on use powers of two, in computing.

The cast to size_t is good since these are sizes, and we want to have them readily usable as arguments to e.g. malloc().

Using the above, it becomes pretty practical to use these in code:

unsigned char big_buffer[MB(1)];

or if( statbuf.st_size >= KB(8) ) { printf("file is 8 KB (or larger)\n"); }

but you could of course just use them to make further defines:

#define MEGABYTE MB(1)