From sea level we look out from the shore to the ocean and see a flat horizon.
Rising up in a hot air balloon we see the horizon still rising with us, especially at the corners of our field of vision. There is no curving away at the edges which should be happening if the Earth was a globe
If we were on a ball-Earth no matter how big, even if it were a million miles in circumference, the horizon of any ball Earth by necessity must remain exactly where it is! A horizon which rises to the eye of the observer can only be an extended flat plane.
If it were a ball, no matter how big, you would have to look DOWN more and more the higher you ascended. Think about it, no matter how big the ball is, if you rose off it in a hot-air balloon and stared straight ahead the whole time, you should be staring off into the “outer-space” beyond the curvature! In reality however, you will be staring directly at the horizon the entire way up without ever tilting your head downwards a single degree. ~ Eric Dubay