Most of my stories and all of my series happen within one of five cities in the United States. What I’m working for here is a sense of continuity: if the mayor of Greenville gets shrunken down and kidnapped, he’ll be missing in the other Greenville stories too. The logic behind how giants and tiny people happen to exist is consistently isolated to these four cities (representing parallel, discrete realities): a woman who magically grows into a giantess will never encounter a natural-born giantess. This is a rule for sanity’s sake, though any rule stands to be broken in extreme cases.
So when you read one of these stories and you see a city named (the states are never mentioned—these escapades could happen anywhere!), you kind of know what to expect in terms of why the men are tiny or the women have grown to impossible size. This helps cut down on the bulky exposition in which the wheel is reinvented for every single story.
This is where technology is responsible for reducing people in size or enlarging them beyond what’s natural. Quantum field bubbles were invented here: within one, you can be reduced to minuscule size, yet retain your strength and endurance and breathe converted air molecules, even when submerged or otherwise smothered. The anarchial Undermountain Labs is hidden somewhere just outside Riverside, inflicting their experiments upon the unwary, and if any aliens descend to Earth with shrink/growth tools, this town will be their base of operations.
This is the realm of magic and divine intervention. Women become powerful by learning spells to increase their height or reduce those around them to playthings. Curses are active and gods regularly interfere with mortal doings in Greenville, and the library is full of books that shouldn’t exist.
A long time ago there was a talented assassin known as Kazimir al’Qazr. An innate master of alchemical studies, one of his greatest inventions was a potion to shrink people down to only a few inches tall. This was always stored in a blue bottle, with its antidote in a purple one. Today, a mysterious pharmaceutical company called Saghir Chemise uses the original recipe to sell this amazing product to desperate, bored people with too much money.
Just like the real world, except there is also a race of Tinies. They were born the size of action figures, and they live in (relative) harmony with the Normals. Some feral tribes hide out in Normals’ homes, camped within their walls and foraging for scraps; others are fully integrated with Normal society and wear subsidized Big Suits™ to work in offices, operating what amounts to a Normal-sized mecha equipped with specialty functions for any job.
Again, like our real world, except giants and giantesses have always existed. The Titanomachy? The Nephilim? Aghartha? Totally real, all of it. Rather than being modified by spells or alien technology, tremendous people were naturally born this way. Sometimes they’re still sleeping in the earth, other times they’re reclusive and remote, and once in a while they go on a rampage. You could look for a friendly giantess, but is it worth the risk?