Building the entire world in Minecraft on a 1:1 scale seems like it should be possible. It's not, though: Minecraft has limitations, most importantly its 256 block height limitation, which won't exactly allow the construction of, say, Mount Everest. But with the use of two important mods, one YouTuber has not only avoided that height limitation, but has also automated the process of mapping the world.
In the video above, PippenFTS explains how. Using the mod Cubic Chunks, it's possible to effectively remove the game's height limitation. Meanwhile, Terra 1-to-1 does what the name implies: it draws from map data, chiefly Google Earth, in order to closely recreate actual real world landscapes.
As a result, PippenFTS claims to have generated a full 1:1 map of the world in Minecraft. Examples used in the video include the Grand Canyon and Mount Everest, and it's definitely true: you've never seen terrain on this scale in a video game. It's not without its problems, though. Human made structures, in cases where their dimensions are recorded by mapping services, make use of normal Minecraft terrain blocks, meaning they don't look as they should - even by Minecraft standards. That means if a more realistic model is going to exist, it needs the help of human hands.
So PippenFTS has started an initiative called Build The Earth. The hope is that hundreds of others will help optimise the map, with each user specialising in their own chunk of the world. There's a Discord server if you want to get involved.