How big is a T. rex?

Well, the animatronic wonder ("Rexy" in in the original Jurassic Park was a good 40 feet long and could tower 20 feet tall. She needed twelve handlers and weighed around 13,000 pounds (even more wet).     Sue, the world's most famous fossil, was probably about the same length and heavier (maybe 18,000 lbs plus) She took 30,000 labor hours to free, clean, and reassemble. She (the gender is not certain) stands about 13 feet high. The Field Museum proudly points out you're seeing the real skeleton, not a cast, although the head on said skeleton is a cast because they couldn't mount the 600-lb skull without the whole thing crashing down.  But the real skull is right there on display. I have not met Sue, but she is on my bucket list for sure.  
Sue starred in one of my favorite fantasy series, The Dresden Files: one of the magic rules in Jim Butcher's series is that zombie are more powerful the older the body or skeleton is.  When zombies attack Chicago, wizard Harry Dresden knows just where to find a skeleton he can animate to mop the city streets with them... 
Then there's Scotty.  Originally found in Canada in 1991 and teased out over decades from hard rock, the skeleton finally assembled (about 65 percent of it is there) indicates an animal that might have been several feet longer than Sue and a thousand pounds heavier.    The "might have been" is important if you think size matters, since we don't have as much of Scotty as we do of Sue, who was 90 percent complete.  And we really don't know how big an average adult was (some think Sue is in this category), or what might have been the size of an outlier.
Just say they were big. But it's sad they're extinct.