OK, I try to elaborate a bit. :) Basically, you can just simply replace a noun with の, so that you can avoid repeating the noun. For example, you can have following dialogue: A: 「この本はあなたの本ですか？」 ("What comes to this book, is it your book?") B: 「いいえ、違います。幸子さんの本です。」 ("No, it's not. It's Sachiko's book") Once you know that it's a book/本 that you are talking about, you can just leave the word out. The の in the sentences acts as placeholder for the word 本 aswell as denotes ownership. Like this: A: 「この本はあなたのですか？」 ("What comes to this book, is it yours?") B: 「いいえ、違います。幸子さんのです。」 ("No, it's not. It's Sachiko's") So, your example could be part of this kind of conversation: A: 「これはCDです。」 B: 「小さいCDですね。」または、「小さいのですね。」 Here you can either repeat the word CD or replace it with の, which means in this case "one" ("It's a small one, right?") or "thing" ("It's a small thing, isn't it"). Another example: A: 「どんな映画が好きですか？」 B: 「面白いのが好きです。」 ("I like interesting ones") You can of course repeat the word 映画 and say also 「面白い映画が好きです。」 Please read more about の in here: http://www.guidetojapanese.org/learn/grammar/nounparticles. Scroll down until you find headline "The 「の」 particle". :)

Leave out the topic particle は and you have a correct sentence. :)