For anyone who has been following this thread, the new UE5 World Partition valid large terrain sizes are calculated by (511 x N) - (N - 1).
For example: 16321, 24481, 28561, 32641, 44881, 48961, etc.
The small single terrains from 127 to 8129 still work the same in UE5 as they did in UE4, those values are still valid also for UE5. However, large terrain sizes greater than 8129x8129 have now changed.
You can no longer go by the UE4 Landscape Recommended Sizes documentation for terrains larger than 8129x8129, as World Partition uses the larger 511 Section size.
So you cannot go by World Composition sizes such as 8x8 tiles of 1009x1009 tile size, with shared edges, because that will result in either a cropped or padded landscape in World Partition.
The same goes for converting UE4 World Compositions into UE5 World Partitions, you will end up with an incorrectly sized terrain, that is either cropped or padded.
So World Partition terrains should be created from new with the proper resolutions.