Earth actually is flat, well, almost.
Over time our science has been able to show that Earth is almost a sphere, pretty much what is called an "oblate spheroid". And yet it is a sphere (almost) that is about 25,000 miles in circumference. That's quite large in comparison to the wee creatures of Earth – like us for example.
A flat Earth would mean zero curvature to its surface, or in other words, Earth's surface curvature would equal 0. We already know that Earth is "almost" flat, so how close is its surface curvature to zero? The answer is that the Earth's surface curvature slightly varies around 0.000126[1] (about 8 inches to the mile).
That figure, 0.000126, is very close to zero as you can see. It's no wonder that the ancients got it wrong and thought the Earth was flat. The head-scratcher is that anyone in our modern age would still think that there is zero surface curvature to the Earth.
Even the ancients saw clues to the Earth's real shape, such as how if you walk at night in a northerly direction, stars would slowly disappear on the southern horizon. And if you turned around and walked south, stars would slowly disappear on the northern horizon. That would not happen if Earth's surface were completely flat. They had other clues, too, like a ship disappearing on the horizon as it sailed away – first the bow, then the stern, and finally the tall masts would slowly vanish. That would not happen if Earth's surface curvature were zero. Just like now in modern times, every so often there was a lunar eclipse, and even the ancients could see the arc of the Earth's surface as it slowly covered the Moon's darkening, reddening sky image. Another more subtle clue was how, in the northern hemisphere, the length of the shadows cast by the Sun would get longer as you walked north or shorter if you walked south. It all seems crystal clear to me. And it's also clear that there are three possibilities: either 1) flat Earthers drink too much, or 2) they smoke too much, or 3) they are simply deniers of reality.
Here's one I haven't read anywhere: everyone along the path of a solar eclipse sees it happen at different times of the day. If the Earth were flat, wouldn't they all see the eclipse happen at the same time?