New user with some questions regarding world building

Hey everyone, I’m Addezz a recently self proclaimed game dev! I’ve had a interest for game development for a couple of years now but never really gave it a go but now here iam!

So with that short introduction out of the way i find myself quite lost! The video tutorials and learn section has been great along with community made stuff from youtube ive been learning quite abit, however i have a problem.
I want to import a island/world and not make one from scratch ( i cant sculp even if my life depended on it) I used worldmachine but as a free used the max resolution is 513x513 and i could never press the “build tile” button. I also had trouble exporting my world from world machine into heightmaps (help pls?)

I then checked out “Twiz’s” youtube video and tried doing it that way. Well this is the what i ended up with Screenshot - eae8ae424ce087cd4a2a438c58362ab3 - Gyazo
So what could be the problem here? Something to do with tiles/quads/components?

My question is how can i get a 24x24km2 world into unreal? I want to make an island.

some programs im using is L3DT pro / gimp 2 and im about to check out blender

Thanks in advance,