Are you talking about 72 points to the inch or 73 points to the inch?
Points are meaningless in the digital era. Cap height or x-height are more accurate.
Ditch points and leading and you will be much happier

@terminaldesign - No doubt, however...

Points to the inch has no bearing to my enquiry.

In my text, I am referring to real physical size (in the pre digital sense) therefore point or points...

This is not about how to describe a measurement but how to define points.

...But in the 'digital era' aren't all cap heights the same?

A 3 inch nail is 3 inches long.

I'd think this distinction would also apply to type sizes.

The first size in your example, "4.5 point", is a specific size. The next, "21 points" is more of a general description than an exact dimension.

In the digital age call 'em what ever you like, they're just units.

-=®=-

I'd say it's a matter of nominal vs. adjectival forms. In adjectival forms with units of measure, you use the singular (e.g., "20-dollar bill"). When used as a noun, the unit of measure takes the plural with numbers other than one (e.g., "it costs 20 dollars"). So A series of seventeen Didot fonts sized between 4.5 point and 21 points is not correct, because both instances are functioning as nouns rather than adjectives, whereas A ‘9 on 12 point body’ describes a type usual to a 9 point body which is cast onto a body three points larger is correct, because it uses singular where the number plus unit function as an adjective modifying a noun, and plural where the unit is used as a noun.

It might be different sometimes in a language that has plural adjectives, assuming it allowed such constructions.

Almost there. The shift to singular happens when the measure is part of a compound adjective that modifies a noun. Compound adjectives should also be hyphenated: a 3-inch nail, an 80-year-old man, 12-point type.

The measure stays plural if it's in a phrase on the other side of a verb. Then it's modifying an adjective. The actual adjective of the phrase is sometimes missing but implied: "The race is ten miles" really means "The race is ten miles long."

Now, some fudging occurs when people start to think of a compound adjective as one that can be used where any other adjective can. Arguably, you can say "The type is 12-point" as easily as the "The type is black." But it's an unusual way to say things in most situations; we don't say "The race is ten-mile."

So let's look at your example.

"A series of fonts sized between 4.5 points and 21 points was founded."
("was", not "were", because the subject is "A series".) I would argue that the word "sized" demands this form. "This font is sized to 21 points" is like "This building was built 21 floors tall."

I think that use of "sized" is awkward anyway. I would say:
"A series of fonts between 4.5 points and 21 points in size was founded."

You could also say:
"A series of fonts from 4.5-point to 21-point was founded."
This treatment of adjectives is similar to that in such phrases as "all creatures great and small".

Above all, measurements listed together as the same part of speech should be in a consistent form, no matter how much more "specific" you think one number is than the other!

If developing a 'house-style' for usage do you think it would be good to differentiate between size (as dimension) and physical state?

[Updated from top] So far when referring to points as a 'physical state' of a (metal) type the number is a figure. [i.e. 7-point type]. (Is referred to in singular, and is hyphenated)

So with the convention of writing out numbers as words in running text helps identify a size (as dimension) [i.e. three points larger] clearly differentiating from the 'physical state' which uses a figure [i.e. 7-point type].

A ‘9 on 12-point body’ describes a type usual to a 9-point body which is cast onto a body three points larger.

Concerning sizes (as a dimension of scale)
...investigating features considered extraordinary or specific to the design of a typeface concerned with reproduction and legibility at sizes less than 7 points.
becomes
... and legibility at sizes less than seven points.

So taking two quotes (that I won't change in the text (as they are are quotes) but will apply the 'house-style' rules here for clarity are as follows:

Quote
The process was reversed for the large sizes, 18 points and larger, where legibility was not a problem and an aesthetic motive could apply. [Walter Tracy]
would become
... for the large sizes, 18-point and larger, where legibility was not a problem and an aesthetic motive could apply.

Quote
Shorted descending and ascending strokes are unforgivable on bodies over 18 points. It is quite legitimate to shorten the tails of the small fonts [lowercase] to increase legibility and to lengthen them in the display size of the same face for the sake of elegance. [Harry Carter]
would become
...on bodies over 18-point. (as is descriptive of the physical body of type)

So far so good?

Does anyone have a perspective on the typography of ranges.
Here I am using an em to signify the range and a dash for the hyphenation.
Does a string of numbers and an assortment look wrong to you?

...copyfitting value much appreciated for types sized between 9—12-point, is diminished at sizes 8—6-point (or smaller) because of its condensed stature.

In the first example, because of the use of the word "between", you should be using the word "and" instead of an em dash there, IMHO. "...types sized between 9- and 14-point,..."

_______________________________________________
Ever since I chose to block pop-ups, my toaster's stopped working.

Yes and yes. I am trying to verbalise technical attributes, so this is both correct and helpful.

...also a correction from before:

Table 01 is a composite of multiple sources, gathering together information specifically directed at type sizes below seven points.
actualy refers to 'type size' and not 'size as a dimension' so should be
Table 01 is a composite of multiple sources, gathering together information specifically directed at type sizes below 7-point.

Say "between 4.5 and 21 points" and you will be fine. There is no NEED to repeat the noun and indeed there's a good reason not to: mere repetition bores. obillo