This depends on which long-necked dinosaur you are talking about. Heights varied between species.
Argentinosaurus was the biggest hervivorous dinosaur that has a long neck.
Amargasaurus.
There were a number of long-necked dinosaurs. One was the brontosaurus.
Yes. this dinosaur stood 11 feet tall and was named the unumbrellasaurus for its large flared out umbrella like neck. no jk
Plesiosaur
grass and leaves
Diplodocus.
A long neck dinosaur that eats plants
Janenschia is a dinosaur. It was a huge dinosaur with a long neck and tail.
it has a long neck
mamenchasaurus
Ultrasauros is a dubious taxon. Specimens once belonging to Ultrasauros now belong to Supersaurus. If paleontologists have the incline of its neck correct, Supersaurus was about 26 feet tall, but 110 feet long.