This is not true.
You should not be using random dimension sizes for Landscape, or “any size”.
For standard sizes Epic has a document that lists what the Recommended sizes are, and you should stick to only those sizes.
For World Partition terrains you should stick to only valid WP sizes.
For example, if you are creating a World Partition Landscape, during the import process, if you are not using valid World Partition dimensions, the editor will pad out your heightmap to a valid World Partition dimension, and that results in a stretched and padded edge to your terrain.
You don’t want that.
So you should always use a valid World Partition resolution.
WP Resolutions are calculated as (511 x N) - (N - 1).
Anything other than that sizing will result in a messed up Landscape.
You should also typically stick with a square size, or you could run into troubles.