>>279612673
That's basically modern shounen anime. In the 90s and earlier, a girl would barely have a C cup and the story would act like she has the biggist tits on the world (even though half the cast was the same size). Now girls with a D cup are mocked for being small and the typical girl is like H cup.
>>279613354
It's just any girl who's significantly shorter than the average. So in Japan it would be like under 5' 2".

However, if you really want to get technical, it's more of a proportional thing. And that's why people can't lock down the shortstack term. It's a girl with some slight dwarfism/childlike proportions, despite being post puberty. Someone with proportionally bigger head/thicker thighs/wider hips/bigger breasts than typical for her height. So you can even have a tall girl who looks like a shortstack, because her head is bigger or her proportions are thicker than normal for her height (see image). But of course we wouldn't want to classify them as a shortstack because it defeats the purpose.