1941 Agatha Christie novel featuring Tommy and Tuppence Beresford (4)

I think it could be either:

Which length description are you looking for?:

1,2,1

norm

(Other definitions for norm that I've seen before include "Rule, standard, pattern" , "Average result" , "Standard level of performance" , "What is conventional, regular" , "Accepted standard of behaviour" .)

I've seen this clue in The Mirror.

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