Using Heightmap Tiles with World Partition importing doesn’t gain you anything, the import process still stitches them all together into one heightmap, so the system memory and GPU memory use and requirements is the same, whether you use tiles or a single file.
If you want a 16km x 32km heightmap, you should be able to import that into UE5.3 simply by disabling Edit Layers on the Landscape import dialog.
If you require Edit Layers, then the current maximum supported size is 16km x 16km since that is the largest DX Texture that can be used for the edit layers.
UE5.3 was supposed to have a “successive batch import” new feature, but I haven’t found out how to use it or where it is, and Epic is terrible at documenting features.
A 16km x 32km heightmap in World Partition valid size is 16321 x 32641.
For UE5/5.1/5.2 you will probably require a computer with 64GB+ memory and a 12GB+ GPU to import that size, unless you are using UE5.3, which uses less memory during importing (Epic streamlined the import process in UE5.3 and it typically uses way less memory during importing). You would have to perform an import test to see if your system can handle it. Any CPU memory allocations in UE will use Virtual Memory (Page File) but GPU memory allocations will only use Dedicated+Shared, so if you run out of GPU memory you are kaput.
If you want to use Edit Layers and you are fine with using a Landscape Scale XY of 200 (2 meters per quad), then you could use an 8km x 16km heightmap with Scale 200,200.
A valid World Partition heightmap of that size would be 8161 x 16321.
Scaling the Landscape up by 2x still usually gives a decent looking terrain system.
Most people don’t use rectangular terrains though, so just make sure whatever software that you are creating the heightmap with supports it. You may have to create a 16km x 16km heightmap and then crop it into a rectangle.
Whether you go with a square or rectangular Landscape might be determined by what your view distance and terrain layout is. There is nothing wrong with using the rectangular middle of a square terrain for the play area, that way you still get a good view distance in all directions. But that depends on whether you have mountains etc in the distance in all directions.
You may also run into issues in Unreal with using a rectangular Landscape, so keep that in mind. It might mess up with other systems or actors that you are using.
That is why most people stick with square Landscapes.
I would use a single Landscape actor of the size that you want.
Creating multiple Landscapes and placing them beside each other has lots of issues, including edge seam matching etc.