"I'm speechless!" (4)

I think it could be either:

Which length description are you looking for?:

gasp

(Other definitions for gasp that I've seen before include "Utter breathlessly" , "Sudden intake of breath" , "Noisily breathe" , "Catch one's breath in astonishment" , "Inhale sharply with astonishment" .)

I've seen this clue in The New York Times.

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