According to Memory Alpha, the Enterprise-D was equipped with a cargo transporter.
What was the largest object or amount of mass that the Enterprise-D could transport on board in a single beam up?
The official Starfleet Technical Manual refers to the maximum capacity of the Cargo transport on a "Class-1 Starship" (such as the Enterprise NCC-1701) as being '2.8L x 2.5W x 2.0H' or '3.45L x 1.0W x 2.0H' in meters.
While this book is now considered to be non-canonical, it was written by several of the principle designers of the Trek Universe and has been used as a source and reference for four Star Trek motion pictures, and at least one episode of TNG and multiple episodes of Voyager and DS9.
The cargo pad on the Enterprise-D appears to be of a similar size and shape so it's reasonable to assume that the constraints on transport size are around the same.
It has been noted that in TNG: Deja Q, at one point Picard orders an entire shuttlecraft (and its human contents) beamed site-to-site into the Shuttlebay. Since a standard shuttle is approx 9.1m x 3m x 4m, it seems logical that he was expecting his crew to tie multiple transporters together and use them in site-to-site mode.
PICARD: This goes against all my better judgment... (keys insignia) Transporter Room Three, lock on to Shuttle One... beam it back to it's bay.
As @Pioneer has pointed out in his comment below, In Nemesis, the Enterprise-E was evidently capable of transporting a Scorpion-class Attack Flier. These don't appear to be much larger than the standard pad above but they're probably the largest thing we actually see being transported in a single pass.
For precedent in canon: Not the largest single object, but Worf's half-brother beamed an entire colony and their trappings into a holodeck.