I'll start with:
Q: "What is the circumference and diameter of the Earth?"
Circumference: 78225 miles
Diameter: 24,900 miles
and
Q: "What about the stars, sun and moon and other planets? Are they flat too? What are they made of?"
A: The sun ... 32 miles in diameter, circle(s) Earth at a height of 3000 miles at its equator ... located midway between the North Pole and the ice wall. (The sun is) similar to a "spotlight." ... As (it is a) spotlight, (it) only give(s) light out over a certain area which explains why some parts of the Earth are dark when others are light.
So the circumference at the "equator" (halfway between the north pole and ice wall) would be ~39,000miles, and we know in a couple of days, during the equinox, the equator will be getting 12h of daylight, and 12h of darkness. The sun will also be seen to cross the sky, rising directly east, moving directly overhead, and setting directly west, carving out what appears to be 180 degrees of angle change in 12 hours. The sun can be observed to set in roughly 2 minutes. This can be tested at any time of the day, as the sun will appear to move the distance of its diameter every two minutes.
Based on this, and the theory of bendy light, we know the light bends upwards from 3000 miles up and during the equinox, at the equator must neccesarily be able to propagate light halfway across the circumference of the equator for an area of ~19,500 miles, ~9,750 miles in one direction. This would be the maximum light can bend, otherwise the sun would be seen from father away.
A few facts based on these data.
Each 1,625 miles would represent approximately one hour time difference.
Two minutes would constitute ~55 miles.

My image's scale is 1 pixel = 20 miles.
The divergence of the line as it approaches the ground is very slight, and almost indetectable, but as it arcs away from the ground the difference becomes more pronounced. The farthest mark on the graph represents 9,750 miles away, the maximum viewable distance of the final sliver of the sun as it sets at sea level, the mark 3 pixels in represents the last point where the entire sun is still visible.
What this drawing implies is that using the principles of bendy light, whenever you look at the sun past sunset by increasing your altitude, what you are actually seeing is an inverted sun.
This can be tested for at sunset by having a friend stand at the top of a hill at sunset, and using a pinhole projector or similar device, draw the sun noting where the sunspots are at the same time as you are drawing the sun at sea level, and compare the two drawings. Photographs could also be used, and there are other techniques for seeing sunspots.
NOTE: Never, under any circumstance look directly at the sun for any reason.