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.
The scientific name of a long neck dinosaur is Brachiosaurus.
Yes. this dinosaur stood 11 feet tall and was named the unumbrellasaurus for its large flared out umbrella like neck. no jk
A long neck dinosaur that eats plants
grass and leaves
Diplodocus.
Plesiosaur
Janenschia is a dinosaur. It was a huge dinosaur with a long neck and tail.
mamenchasaurus
it has a long neck
Diplodocus was a long-necked whip-tailed dinosaur measuring about 90 feet long with a 26 foot long neck and a 45 foot long tail. It could reach up to 35 115 feet tall, and over 20 ft of that was its neck.