sumGirl was telling us:
sumGirl nous racontait que :

This reply was meant, I think, to show you that what you are asking is
nearly impossible as soon as you work with real document (Tables, sections
breaks, hyphens, page breaks, punctuation, etc.)
And what if a word is found 200 times? And in the same document another is
found only once? You will have 200 hundred words at font size 200 and one at
font size 1?

In any case, here's a little something to get you going.
You are re going to need to add error trapping of all sorts and try to
foresee all kinds of undesirable stuff that may end up in your document that
should be ignored.
I put the entire doc in a string otherwise the processing would take too
long...

'_______________________________________
Sub ProcessWords()

Dim strDoc As String
Dim strDocArray() As String
Dim i As Long
Dim k As Long

strDoc = ActiveDocument.Range.Text

strDoc = Replace(strDoc, Chr(13), " ")
'Remove all double spaces from string
Do While InStr(1, strDoc, "  ") > 0
   strDoc = Replace(strDoc, "  ", " ")
Loop
strDoc = Replace(strDoc, " ", "|")
'Convert to lowercase
strDoc = LCase(strDoc)
'Remove trailing "|"
Do While Right(strDoc, 1) = "|"
   strDoc = Left(strDoc, Len(strDoc) - 1)
Loop
Do While Left(strDoc, 1) = "|"
   strDoc = Right(strDoc, Len(strDoc) - 1)
Loop
strDocArray = Split(strDoc, "|")

WordBasic.SortArray strDocArray()

k = 1
i = 0

For i = 0 To UBound(strDocArray)
   If i < UBound(strDocArray) Then
       If strDocArray(i) <> strDocArray(i + 1) Then
           ReplaceFontSize strDocArray(i), k
       Else
           k = k + 1
       End If
   End If
Next

'One more time as the last one was not done
'because we got to the end of the array
ReplaceFontSize strDocArray(UBound(strDocArray)), k

End Sub
'_______________________________________

'_______________________________________
Private Sub ReplaceFontSize(ByRef strWord As String, ByRef lngFontSize As
Long)

With ActiveDocument.Range.Find
   .Text = strWord
   .Replacement.Text = strWord
   '5 is used so that we have a minimum font that is legible
   .Replacement.Font.Size = 5 + lngFontSize
   .Execute Replace:=wdReplaceAll
   lngFontSize = 1
End With

End Sub
'_______________________________________