They indicate size of trotters (4)

I believe the answer is:

feet

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'trotters' is the definition.
(trotters are pigs' feet)

I don't understand how the rest of the clue works.

(Other definitions for feet that I've seen before include "Measures - body parts" , "On which we stand" , "Old measures - that fill shoes" , "Linear Units of a third of a yard" , "Body parts; lengths" .)

I've seen this clue in The Irish Times.

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