Most if not all string algorithms will eventually do this anyway:

  size_t length = strlen("some string");

It's so common. Might as well memoize it so it's always available with no need to constantly loop through strings which is an O(N) algorithm. So many string algorithms call strlen, often multiple times on the same string. I remember GTA V took 6 minutes to parse a goddamn JSON file because of stuff like this and part of the fix was to store the string lengths.

https://nee.lv/2021/02/28/How-I-cut-GTA-Online-loading-times...

So is a length variable really such a big deal? It even fits in a register.

I understand and agree with your one memory span point. Ideally they should be located as close as possible in memory.

  struct bytes { size_t size; unsigned char data[]; };

  const char *literal = "some text";
  size_t length = strlen(literal);

  struct bytes *text = malloc(sizeof(*text) + length);
  text->size = length;
  memcpy(text->data, literal, length);