was it big enough for the dog to move around in?
Yes, it took up the entire bed of the truck. Usually Missy stands when I drive her around and she's fine. She stands so she can stick her head out the window.

Maybe I'm totally wrong, but in my simple mind dogs have more balance because they have four legs on the ground. LOL to me more legs means more balance. Does anyone actually know if they do or not. Because like you said I have a hard time standing on a bus, but Missy can stand on my car seat just fine.