The answer is HALE, four letters. American grids reach for it first because it is short, vowel friendly and almost extinct in everyday speech, which is exactly the profile setters like. WELL also fits four squares, FIT covers three and SOUND takes five, so count the blanks before you write anything in ink.
Nobody says hale except in one pairing
Try to remember the last time you heard hale used on its own. It arrives welded to hearty, and that pairing is doing all the work of keeping the word alive. English collects these doubled phrases, null and void, hue and cry, fast and loose, where one half stopped being useful on its own centuries ago and rides along inside the set phrase. Crossword grids are one of the few places where the stranded half gets out on its own.
Its narrow range is exactly what makes it useful to a constructor. A word that shows up in daily conversation carries too many associations and gets clued the same way every time, while hale arrives with one obvious definition and no baggage, which lets an editor drop it into a corner without the entry feeling tired. It also has the letter pattern editors want, an alternating consonant and vowel run that plays well with whatever crosses it.
Be careful with the other hale, the verb meaning to drag or haul someone somewhere, as in a defendant haled into court. It is a separate word with a separate history, and a clue built on it wants a completely different sense. If the wording involves force or a courtroom rather than health, you are looking at the verb.

Six words for well, and the squares each one takes
| Letters | Answer | Notes |
|---|---|---|
| 3 | FIT | Leans athletic rather than simply not ill |
| 4 | HALE | The standard fill for plain wording |
| 4 | WELL | Competes directly with HALE |
| 5 | SOUND | Often clued about a structure or an argument too |
| 6 | HEARTY | Also means generous, as in a hearty meal |
| 6 | ROBUST | Strength more than the absence of illness |
| 9 | INTHEPINK | Long across entries, from the old idiom |
Sometimes healthy means large, not well
A healthy raise, a healthy portion, a healthy lead in the fourth quarter: none of those involve a body at all. The sense is generous or substantial, and setters use it constantly because it flips a clue that looks medical into one that is not. That opens up AMPLE at five squares, TIDY at four, as in a tidy sum, and BIG at three. TIDY is the real trap, because it takes the same four squares as HALE and WELL, and nothing in the bare clue tells you which sense is live. Only a crossing letter settles it.
The giveaway is usually the neighborhood. If the clues around yours are about money, servings or margins, read the clue as generous. If they are about bodies, doctors or exercise, read it as well. Editors group their misdirection loosely, so a single clue almost never fights the theme running through the rest of its corner.
The crossing that separates HALE from WELL
These two share exactly one letter in one position, the L in the third square. Every other position differs, so a single crossing entry at the first, second or fourth square resolves the whole thing. That makes the down clues around a four letter healthy far more useful than any amount of staring at the clue itself. Work one of them, take the letter, and the choice makes itself.
There is a tiebreak worth knowing for when no crossing is available yet. WELL is far more common in speech, which means editors more often clue it through its other jobs, as an adverb or as the thing you draw water from, and save the health sense for when the grid needs it. HALE has nowhere else to go. When the wording is a plain adjective carrying no wordplay at all, the odds favor HALE.
Healthful and healthy used to split the job
Usage guides once drew a clean line between them. Healthful described the thing that promotes health, a healthful diet, while healthy described the one that possesses it, a healthy child. Ordinary American English has largely merged the two, and most readers now use healthy for both. Crossword clues occasionally exploit the older split, so a clue reading “Healthful” may be after something like WHOLESOME rather than a synonym for well. It is a small distinction, but it is the kind that decides an eight letter slot.