正好 (zhènghǎo) can be used as an adjective or adverb to help express a coincidence, and it could be related to size, volume, quantity, degree, time, etc. This is similar to how we say in English “Oh, this shirt I grabbed was just the right size." or something like that.

Using 正好 with a noun
When used with a noun, use 正好 right after it, followed by an adjective.

Examples

Zhè jiàn yīfu de hàomǎ zhènɡ hǎo.
这 件 衣服 的 号码 正好。
The size of these clothes is just right.

Using 正好 with a verb
With just the verb
When used with a verb, the 正好 goes right before it.

Examples

Wǒ chū mén de shíhou zhènɡhǎo xiàyǔ.
我 出 门 的 时候正好 下雨。
I was going out the door just as it started to rain. 

 With just 得 and a verb
You can also use 正好 along with 得, as shown in the following examples. This is usually a clause of its own.

Examples

Nǐ lái dé zhènɡ hǎo, bú zǎo yě bú wǎn.
你 来 得 正好，不 早 也 不 晚。
You arrived at just the right time, not too early and not too late.