Choosing the number of Rows, Columns, and Layers as well as their dimensions can either be a very simple task or a difficult one.
If you are using Unity Terrain assets or some other type of 'base/ground' asset, then the number of Rows/Columns (remember, it's only possible to use a single Layer with Terrain/Ground Meshes) will typically match the number of assets you want to place on each of those axes, while the size of each Row/Column will match the size of the assets themselves. That is to say, the borders of each Cell should line up with the borders of the Terrain/Ground Meshes, so that the terrain/mesh fills up the entire space of the Cell.
If not using an asset that fills the entire space of the cells they are in, then choosing the number of Rows/Columns/Layers will be more difficult. Note that most assets will fall into this category, so it's important to pay attention to the following notes when making your decisions:
1) If your assets can be split up into multiple Streamable Asset Chunks, do not choose cell sizes with the intention of trying to reduce the number of assets per cell, since you can reduce the overhead of load operations by using multiple chunks rather than smaller cell sizes.
2) Generally speaking, larger cell sizes will result in faster updates of the world, as the Streamable Assets Manager will have less Cells to process. However, larger Cell sizes mean assets farther away from the player have a higher chance of staying in the scene, which will increase the memory used by your game.
3) Smaller Cell sizes may slow down the overall speed at which the world is updated, but offer more flexibility in the number of assets that are loaded at any given time.
4) Your player's movement speed should be considered when choosing between small and large Cells. The faster the player moves, the larger your Cells should be, as smaller Cell sizes may slow down the update speed of the world sufficiently that it cannot keep up with the change in position of the player.
5) You can modify the size of your axes later, however any Assets already assigned to Asset Chunks (which are in turn assigned to Cells) will need to be reassigned. While the World Designer tool can aid you in this task, it can still take a while to complete this task if your World is very large; as such, it's best to try to nail down the sizes of your axes as soon as possible, before you have proceeded too far with building your World. In other words, try building a prototype world if you are unsure of the Axes sizes/Number of Cells, before creating your final World.
As you may or may not be aware, you can combine multiple Streamable Grids together under a single World, by utilizing multiple World Groupings.
When doing so, the size of each Sub World Grouping's Streamable Grid must match the size of the Base World Groupings Streamable Grid. That is to say, the total width (added width of all Columns) and the total length (added length of all Rows) of each Streamable Grid must be equal.
The total height (added height of all Layers) must only be equal when both the Sub World Grouping and Base World Grouping are using Streamable Grids with the 3D Axes Type.
--Special Note--
The width/length/height of individual Columns/Rows/Layers do not need to be the same across combined Streamable Grids. It is only the total width, length, and (sometimes) height that have to be equal!