正好 is extremely common in daily conversation. Its usage can be broken down into two main categories:
=== 1. As an Adjective: “Just Right” ===
It's used to describe a noun (like size, amount, length, temperature) as being perfectly suitable.
e.g., 这件衣服大小正好。 (zhè jiàn yīfu dàxiǎo zhènghǎo) - The size of this shirt is just right.
e.g., 汤的温度正好。 (tāng de wēndù zhènghǎo) - The temperature of the soup is perfect.
=== 2. As an Adverb: “Coincidentally / Happen to be” ===
This is its most common function. It's placed before a verb to indicate that an action happened at a very opportune or coincidental moment.
e.g., 我正要出门,正好你打电话来了。 (wǒ zhèng yào chūmén, zhènghǎo nǐ dǎ diànhuà lái le) - I was just about to leave, and you happened to call at the perfect time.
It can also introduce a convenient situation.
e.g., 你要去超市吗?正好,帮我买瓶可乐。 (nǐ yào qù chāoshì ma? zhènghǎo, bāng wǒ mǎi píng kělè) - Are you going to the supermarket? Perfect, help me buy a bottle of coke.