Calculating Heightmap Dimensions
As you can see, the dimensions for a Landscape are based on the number of quads in each section, the number of sections in each component, and the number of components present in the Landscape. Once you determine the number of components and the resolution of each of those components, then calculating the dimensions of the Landscape as a whole becomes a trivial calculation.
Here are some example scenarios:
Example 1
If we start with a component comprised of a single section that contains 64x64 vertices, then the component size is 63x63 quads. Say we have a Landscape of 10x10 of these components, then we have 630x630 quads total in our Landscape. Now if we wanted to import the heights for such a Landscape, we would have to have a heightmap of 631x631 vertices, because there is always one more row of vertices than there are quads (think of a 1x1 quad - it needs 4 vertices). So 631x631 is a valid Landscape size.
Example 2
If we have a component divided into 4 subsections, each of which is made up of 64x64 vertices. That leads to 63x63 quads per section and 126x126 quads per component. If we have 32x32 of these components, we get 126 * 32 = 4032 quads total in each direction. So the total Landscape will be 4033x4033 vertices.
These examples focus on square Landscapes. However, you can create Landscapes that are not square. For instance, in the first example the 10x10 is not special. Assuming 63 quads per component, you could have any Landscape of AxB components with total size in vertices of (A63+1 , B63+1).