Per my answer here, 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.
The Bird of Prey contain several transporters and these can be used in "site to site" mode to transport a very considerable volume if they're all tied in together and used at the same time.
As far as the largest transport seen on screen, in the episode "Distant Origin", 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.
