Why are 成语 (idioms) four characters long? Are there 成语 with more or less than four characters? I really enjoying learning 成语 (chéng yǔ) in Chinese, and I'm curious to know why they are often or always four characters long? Where does this tradition stem from? 

And are there 成语 with more or less than four characters?
Taken from my.chinese.cn:


  成语为什么四字者为多？这大概是因为四字容易上口。如我国古代的诗歌总集《诗经》，就以四字句为多，古代历史《尚书》，其中四字句也有一些。后来初学读的三、百、千《三字经》《百家姓》《千字文》，其中后两种即全为四字句。《四言杂字》《龙文鞭影》初、二、三集，都是四言。这虽然是训蒙书，也足以说明四字句之为人所喜爱、所乐诵。


Why are idioms mostly four characters? This is basically because four words are catchy. Our country's famous book of poetry, 《诗经》, mostly uses four-character sentences, and the historical book 《尚书》 also has many four-character sentences. Then there are the (三、百、千) 《三字经》《百家姓》《千字文》, the latter two of which use four-character sentences. 《四言杂字》《龙文鞭影》 1st, 2nd and 3rd collections all have four words. Although these are books for educating small children they also illustrate people's enjoyment of four-character sentences.

You can find some examples of longer idioms at Wikiquote:


  
  天高皇帝遠。
  Tiān gaō huáng dì yuǎn.
  "The sky is high and the emperor is far away" — used as in carefree (lawlessness).  
  自助者天助。
  Zì zhù zhě tiān zhù
  Literally: Those who help themselves, God will help. Meaning: God will help those who help themselves.
  


You can find some 3 character idioms (in Chinese) at chengyu.itlearner.com.