Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you to remember the ちい portion:
When you think of the word small, try to think of this cheese (ちい) store. You go inside, and all of their cheeses are so small. They hand you a magnifying glass to see all the cheeses. You look at them and exclaim: "Why are these cheeses so small???"
These clothes are small on me.
In this class, there are kids who are still small while there are kids who are as tall as adults.
My ideal marimo would look like a small onion.