How To Say "Small" In Chinese?

Most Commonly Used Words For "Small"

小 is one of the first words you learn in Chinese. It means “small”, “little”, “minor” and is used in a wide array of contexts. 
这只狗很小。– zhè zhǐ gǒu hěn xiǎo – This dog is small. 
在我的小的时候 – zài wǒ de xiǎo de shíhòu – when I was small (young) 
这个机会是很小的。– zhè ge jī huì shì hěn xiǎo de – This opportunity is small. 

Another popular way of describing something small is by negating the opposite. You can just say that the object is not big (不大). 
不大的狗 – bú dà de gǒu – small dog 
不大的机会 – bú dà de jī huì – small opportunity, little chance

Synonyms of 小

微小 and 细小 both mean "tiny", "extremely small". 
微小的尘埃 – wéixiǎo de chén'āi – tiny dust 
一个很微小的改变 – yīgè hěn wéixiǎo de gǎibiàn – very slight change 
细小的杯子 – xìxiǎo de bēizi – small cup 
细小 may also stand for "trivial", "not very important" 
细小的问题 – xì xiǎo de wèn tí – minor problem 

The meaning of 细微 is "tiny", "subtle". It is very close to that of 微小 and 细小. 
细微的变化 – xì wēi de biàn huà – subtle changes 

纤毫 means "slight", "tiny", "very detailed" 
纤毫的差错 – xiān háo de chā cuò – slight error

Idioms

蝇头- yíng tóu – small, tiny (as a fly’s head) 
蝇头小利 - yíng tóu xiǎo lì – very small profit 

微乎其微 – wēi hū qí wēi – almost negligible, next to nothing 
粒子的总体积是微乎其微的。 – lì zǐ de zǒng tǐ jī shì wēi hū qí wēi de - The volume of the particles is negligible.