"The letters in this newspaper are too small."
Translation:Huruf di koran ini terlalu kecil.
It seems to me that you use double-word plurals if you are talking about a specific set of plural things, often owned by someone or something. If you are talking about a thing in general, you can keep it at single word. Contrast: My bunnies eat grass = kelinci-kelinci saya makan rumput Vs. Bunnies eat grass = kelinci makan rumput
In this case, you pretty much know that a newspaper's content is made up of more than one letter, so we are generically talking about the letters in the newspaper.