The French definition of the meter can also be described as "such that the distance from the equator to the North Pole, through Paris, is 10,000 km." This is a handy number to know!

It means, for example, that the circumference of the Earth is four times as big, or 40,000 km (roughly speaking; I know that the Earth is smooshed due to its rotation etc.). And then to estimate the diameter, you just divide that by pi:

40,000 km / 3.14159... ~= 12,732 km

and half that is the radius of the Earth:

r_Earth ~= 6366 km