Hmmm, I think the two examples cover it quite good.
In short: you can’t.
The number of vertices in each of your landscape section needs to be a power of 2 due to mip mapping (you have the same restriction in any texture you use in ue4 they too need to be a power of 2 in order for mip mapping to work). That’s the restriction.
What you further need to consider, is that two adjacent sections share a row of vertices in your heightmap (those shared vertices get duplicated when you import the heightmap, but they are relevant for your heightmap size cause there you would have to subtract them). So if you would have 10x10 components with each 1x1 sections each with 64x64 vertices, so that would be (6310+1)x(6310+1) = 631x631 resolution.
As for the +1, imagine that each section shares a row of vertices in your heightmap with it’s right neighbor, only the section furthest to the right does not have a right neighbor, therefore there you need to count all 64 vertices in your heightmap resolution. So you could also write it down as
(639 + 64) x (639 + 64) = 631x631
Again, the restriction you have is that the number of vertices in each landscape section needs to be a power of 2.
I think the confusing stuff is that the adjacent vertices get duplicated when you import the heightmap and therefore you have to subtract those in your resolution and this leads to confusion