I believe the Imperial Star Destroyer is the biggest in terms of size, at 58.8 cm by 50.5 cm by 21 cm, and the Death Star in the number of pieces, at 3803. Are there any newer, bigger models?
The Super Star Destroyer, with 3152 pieces, is nearly 50 inches (124.5 cm) long and weighs nearly 8 pounds (3.5kg):
While the Ultimate Collectors Edition Millennium Falcon had over 5000 pieces, and was 33" long, 22" wide and 8" tall! (84cm long x 56cm wide x 21cm tall), and was officially advertised as the biggest set ever made - it was almost exactly to Mini Figure scale as well.
In terms of height, I think the Eiffel Tower is unmatched, standing at 1 meter tall on a base of 50x50cm. When looking at the picture below, keep in mind a cat can easily pass under the arches:
The parts count, at 3428, isn't too bad either.
The set with the most pieces as of 2018 is the #75192 Millennium Falcon with 7541 pieces.
It measures over 8” (21cm) high, 33” (84cm) long and 22” (56cm) wide.