Is this a metonymy or synecdoche: "His ears were long."?
I am not sure that its either.
"Lend me your ears" would be a synedoche as it is using a part (ears) of a whole (body/person) to represent the whole.
Metonymy is using a word or words to represent something it is closely linked to. An example would be "No 10 refused to issue a statement" meaning the Prime Minister or government refused to issue a statement, not the building.