Should "The pencil is short" be an acceptable translation? It departs more from the meanings of the Korean words, but adheres more closely to how an English speaker would describe a pencil.
It could be either singular or plural. Korean doesn't make a distinction, unless you have 들, in which case it's definitely plural. Whether it's plural would be understood from context.
What’s the difference between 은 and 이 usage when it comes to sizes adjective? I saw them using pencils as an example for 연필이 짧습니다, and then here it’s 은. I know that 은 is a topic particle and 이 is subject, how does it matter what you use in a sentence?