differential geometry - Triple-right triangle experiment: what's the minimum distance?

As I had showed in a previous answer, among the other ways to prove that the Earth is round, we have the triple-right triangle.

The idea is simple:

1. Starting from point A you move in a straight line for a certain distance.


2. At point B, turn right 90° degrees, move along the line for the same distance.


3. At point C, turn again to the right and do the same.



4. We'll eventually get back at the starting point: point A and C are the same location, thus we just created a triangle with 90° degrees.

This proves that that Earth has a spherical shape (not a perfect sphere), since these movements would only create a square with three sides if we were to do it on a flat surface.

However, the "problem" of this experiment is that it's not really doable on a small scale. The distance must be so much that the curve of our planet can be taken into consideration. Walking 1 meter, then one meter and then another meter won't create a triangle, since the curve of the planet is not that strong.

So my question is: what's the minimum distance we'd need to travel for this experiment to work?