I did a bit of math and I found that Minetest has a world size roughly about one-fifth of the size of Earth, assuming 1 block = 1 cubic meter. I just wanted to post this because I thought it was pretty cool.
If I got my math right, the surface area of a flat MT world would be about 3600 km^2, while Earth's surface area is about 509,000,000 km^2. This means that an MT world has about 0.0007% of the surface area of the Earth, or that you would need about 142,000 MT worlds to equal Earth.
According to www.minetest.net, there are 62,000 ^ 3 blocks in a world. That's 238,328,000,000,000 blocks, or 238,328,000,000,000 cubic meters, which is roughly one-fifth of the Earth's volume, which is 1,080,000,000,000,000 cubic meters. At least that's what I got.