Per my answer [here][1], the largest object that a **Federation Transporter** can transport without special engineering assistance is 2.8L x 2.5W x 2.0H or 3.45L x 1.0W x 2.0H, equivalent to a [large cargo container][2]. 

The Bird of Prey contain several transporters and these can be used in "[site to site][3]" mode to transport a very considerable volume if they're all tied in together and used at the same time.


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As far as the largest transport *seen on screen*, in the episode "[Distant Origin][4]", the U.S.S. Voyager (which as Morgan points out in his comment above is a 344m long x 130m wide x 63m high, 700,000 metric ton Intrepid-class starship) is transported inside a massive Voth city-ship;

> **KIM:** *No response. They're locking onto the ship with an energy beam. It's cutting right through our shields!* **Some sort of
> transporter.** 
> 
> **JANEWAY:** *Report.* 
> 
> **KIM:** **We've been beamed inside the alien vessel.**

![enter image description here][5]


  [1]: https://scifi.stackexchange.com/questions/47246/what-was-the-largest-object-enterprise-d-could-transport-onboard
  [2]: http://www.chesterfieldskips.com/wp-content/uploads/2012/02/8-yd-Builder1.jpg
  [3]: http://en.memory-alpha.org/wiki/Site-to-site_transport
  [4]: http://www.startrek.com/database_article/distant-origin
  [5]: https://i.sstatic.net/b4FUm.jpg