You would need to calculate the length of each paragraph before sorting. The simplest way to do the would be to add all the paragraphs to a table, insert an empty column in the table, calculate each paragraph's length and write it to the empty cell on that row, sort the table by the column you added, then optionally, delete that column and convert the table back to text.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]