Brachiosaurus used to he considered the tallest of the dinosaurs. It was 25 m long, but it stood more erect than other sauropods because its front legs were so much longer than the rear ones. Its head was very small, but it reached up to 13 m above the ground.
Recently, another dinosaur called Sauroposeidon has been described. This huge animal stood 17.2 M high, making it three times the height of the tallest giraffe. Each of the vertebrae in its neck was more than 1 m long, and its bones were filled with tiny air cells which helped to reduce their weight.
Picture credit: google