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
Diplodocus.
Plesiosaur
A long neck dinosaur that eats plants
grass and leaves
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.