Chinese grammar:Coincidence with 正好"zhenghao"

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.
noun.+正好（as a 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.
正好+V.(as a adverb)

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.
V.+得+正好（as a adverb）

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.