As title says, I have no tools nor knowledge for such a thing (tools having a heightmap editor feature tend to kill my pc).

What I'd like is if someone can enlarge Vvanderfell and Solstheim only in their horizontal wideness without changing their altitude? Is that possible?

Vality is the guy to ask. Though the versions that he shared semm to have the height increased as well. You could also upscale the existing heightmap, but I'm pretty sure that such a method would create ridges all along the heightmap.

i can do it for you.
edit: or i can tell you, seeing as it's a pretty easy job. lightwave has a tutorial on it.