The term kodachi (kanji: 小太刀) means "small tachi sword" in English. It is written こだち in hiragana. It's pronounced roughly like "koh dah chee".
Literally, small tachi (the average tachi sword is more than 60 cm in length, but the kodachi is between 30 and 60 cm). Tachi, for its part, literally means fat katana. The kodachi tends now to be confused with the wakizashi.
小, by itself, can be pronounced ko, sho, or o. You will find it, for instance, in the Japanese word for elementary school (shogakko, 小学校, しょうがっこう) and stream (ogawa, 小川, おがわ). The Japanese reading, with the appropriate hiragana characters after it, is chisai (小さい).