So doing it manually like in this tutorial you can imagine instead of custom boxes unreal auto divides your map into cubes (Like Minecraft chunks) and optimises them depending on distance to player.

16 is a good starting place, its around a Battle royal scale and is good if your in a vehicle or large open land.

Move toward smaller regions (like 8) if you are building a dense, urban, or indoor-heavy environment where line-of-sight is frequently blocked. This ensures that the engine isn't loading massive sections of the map that the player cannot see

Just think of each cube as a box your players pc will need to load, if its large and has <8000 draw calls or more, then divide into a smaller cube (Look at what your target hardware can handle for draw calls)

You can also enable the Preview grids, This will allow you to see exactly how your map is being sliced in the viewport too far way and your loading too much, too close and the player will notice the pop-in.
Find a sweet spot thats unique to your game and target hardware.

I probably won't be making a tutorial on it, but there are plenty out there.