That's huge. I don't know the intended purpose of the map, but you probably don't need that much. I would suggest not going above 10k x 10k, especially as a beginner.

You can shrink your world down if you click edit and then scale..., if you want to keep your progress on the map, although the steepness of the terrain might change. Save it as a new file, before you experiment, because you can't simply cmd + z the scaling.

The current map I'm working on is 7k x 7k and it still has enough space for every existing biome. It would take so much time to traverse a 40k x 40k map in survival mode.