RE: Small characters being able to be as tall as Medium ones, I think this is really a non-issue. I can’t imagine anyone who actually wants to play a Small character choosing to be more than like 4’, maybe 4’6” tops, unless they’re just being deliberately contrarian. This is a problem that only exists in the heads of people who actively trying to poke holes.
I'm not an optimizer, at all, but I can see someone declaring their character to be both capital-S Small size for any perceived game advantages, yet tall enough to argue with their DM that they can override any story-based disadvantages of being lowercase-s small. Having height/weight ranges for at least generic "Small" and "Medium" characters would be a nice minimum to avoid such silly but frustrating potential issues.
If they don't plan to concern themselves with differentiating Small and Medium PCs in any lore sense, then they might as well just eliminate Small-sized PCs and be done with it. If we can have Medium characters that are basically Large characters, why not also have Medium characters that are basically Small characters? Give them a "Diminutive Build" trait or something. (This wouldn't be my preference, to be clear, but it would avoid the issue.)
I’d be surprised if the anniversary PHB doesn’t have about as much information on each race in it as the current PHB does. But, I do expect that information will either focus primarily on non-cultural information (because race is not culture), heavily emphasize that it’s talking in generalities and your character isn’t beholden to them, or some combination of both.
I'd hope they do the latter (generalities with an emphasis on your character being whatever they want). Even better if they describe multiple possible cultures, which players could implicitly mix and match when building their own character background; that should be easy for Wizards to do, since they already have multiple cultures for most races.
Standard human gets +1 to all ability scores, so there’s no problem because it doesn’t shoehorn them into any particular subset of classes. Variant human I imagine will lose the bonus proficiency and language and get the same floating +2/+1 or +1/+1/+1 as everyone else plus a Feat.
I just noticed this:
When you create your D&D character, you decide whether your character is a member of the human race or one of the game’s fantastical races, which include the races presented in this article.
So you may be right, humans will be treated completely separately from non-human races, and quite possibly will have their own rules for ASIs instead of the standard +2/+1 or +1(x3). I suppose that's one solution. That would also give them leeway to keep Variant Human largely the same as before, so long as they define it as cultural influence (which I imagine most folks assumed anyway).
You know, I would not be the least bit surprised if those defaults are in the anniversary edition monster manual. Doesn’t the current monster manual have a table for applying race features to NPCs? That seems like the best place for “default” racial ASIs to me.
The DMG is where the racial features for NPCs are located, at least as of 2014 5E. I suppose that could be one place to put ASIs in 2024 5E. However, many players will never read a book outside the PHB, so the convenient location for that information - that is, the place it would be useful for quick-build purposes other than "optimize for your class!" - is still the PHB.