Hi there, Alexandra!
Thanks so much for bringing your question to the Disney Parks Moms Panel. I'm glad you are already planning the details of your Walt Disney World Resort vacation so you can be prepared when you arrive!
Avatar Flight of Passage in Disney's Animal Kingdom is one of Walt Disney World Resort's most popular attractions, but as you have been made aware, the seating for this ride is unique compared to other ride vehicles in the parks. While there are not certain calf dimensions for the most comfort, I'm happy to let you know that a sample ride seat is located near the entrance of this attraction. Here you can check if the seating will be comfortable prior to entering the queue and boarding the attraction. I know many people that have had seating concerns who really enjoyed being able to test this out before entering the queue.
While I have known some people who were fine to ride and others who had to stay back, both still were able to soak up the amazing atmosphere and experiences in
Pandora - The World of Avatar. You will absolutely love the details, music, and scenery in Na'vi River Journey. No matter how many times I ride this, I'm always spotting something new. I also just love walking slowly around the land to take in all the sights and sounds of the animals and plant life that make up the flora and fauna here. Plus you may even be able to catch some local musicians putting on a show. I also absolutely love dining in Pandora - The World of Avatar. From mouth-watering snacks like Pongu Pongu's Lumpia to the delicious and healthy bowls at Satu'li Canteen, there's something for everyone.
I hope this helps, and I can't wait for your journey to this amazing land. Sivako!
-Jennifer