1) The horizon always appears completely flat 360 degrees to the observer, regardless of how high you go up. Any curvature you think you see is from curved airplane windows or Go Pro cameras and fisheye lenses (which NASA loves to use). The reality is that the horizon never curves because we are on an endless plane. On a globe with 25,000 miles in circumference you would see a noticeable disappearance of objects the further they are as they would be leaning away from you and dropping below the constantly curving horizon!
Completely flat horizon from the stratosphere: http://m.airpano.com/360Degree-VirtualTour.php?3D=Stratosphere-Caucasus
2) The horizon always rises to meet your eye level never no matter how high in altitude you go. Even at 20 miles up the horizon rises to meet the observer/camera. This is only physically possible if the earth is a huge "endless" flat plane.

If Earth were a globe, no matter how large, as you ascended the horizon would stay fixed and the observer/camera would have to tilt downward, looking down further and further to see it.

Horizon always at eye level: https://youtu.be/8twAg6CH2sM