No one knows except God. I have a lemon tree. The fruit is easy to pick. I had an apple tree, and my neighbours a mandarin tree. The fruit from both is easy to pick. I do believe that Adam could fly. How else could he subdue all of creation? So I doubt that it mattered how high the trees were. Just fly up and grab what you want.
There is genetic limit to how tall a person gets. I also doubt that Adam had to face dinosaurs and dragons. I believe that they were part of the creation prior to Adam. God made all things good. Why would Adam have to fight them even if they existed? He had authority over them.
Interesting thought about Adam possibly being able to fly.
Notice the curse related to the serpent. Some people think the serpent had legs prior to the Fall, but I have always believed that the serpent had wings. Winged and feathered serpents are part of the ancient mythologies of many peoples around the world. If serpents had legs that God took from them as part of the curse (making them crawl in the dust) then how were they vastly different from lizards, skinks, and similar animals?
Could humans fly originally? Maybe. I know we have a tendency to dream of flying and to want to do it, humanity in general, and also myself personally. Not sure where that longing comes from. I do believe we will fly in eternity. Kind of hard to get around both a new Heaven and Earth otherwise.