Judging by
fans, the Tolkien Hobbit actors seem to be between 4'5 and 4'11. But Tolkien describes the Hobbit as between 2' and 4'.
For D&D purposes, a Halfling can easily be both "Small" and "Medium (Lightweight)", conceptually.
So, the player can choose whether they want a Small or Medium Size Halfling, depending on the abilities that they want to assign. If the hardier Halflings tend to be the ones who who venture, that is probably fine. And if a character is truly massive, then there might be an atavistic expression from certain obscure ancestors who were Dwarf or Human or so.
Meanwhile, use the Size catagories as vague ballparks, and let the player decide what the Size means exactly for ones own character.
Here is an other version of the SIZE PREREQUISITE table. Same idea, but more ballpark and less granularity. It is more straightforward. It gives more wiggle room for players who want a Small character.
If both STR and CON are at least: your Size is
1: Tiny
6: Small
11: Medium
16: Large
21: Huge
26: Gargantuan
Note, a Tiny Pixie is not a normal player character option, and would unusually have either Str or Con be a score of 5 or lower − probably 5 Strength since players are loath to give up Constitution. Typically, these Pixie characters would be poor at classes that require Strength (but who knows, perhaps some Dex build works well), but would be fully competent at caster classes, generally speaking, that can dump Strength.
Besides the extreme outlier of a Tiny character, the player generally decides the Size of the character by choosing the stats.
Generally, Size (Str-Con) ≈ muscle mass (ignoring obesity).
But I hate guess-timating weight during gameplay, and would never do it. However, I would keep muscle mass in mind when deciding the flavor for a particular character.
The OneD&D Players Handbook can, when referring to the Abilities, show pictures as examples of what Sizes can look like, with variations, some shorter and burlier and some taller and lankier.