Need help in creating a map from scratch!

Hello!

In advanced, I appreciate any who read or give an interest in helping me solve my conundrum, per-say.

As per the topic subject, I am really asking for help in this as I am on the verge of starting a full development for a game with roughly 5 people. So this post is as serious as it is helpful to me and possibly others. Other than this, I went through many posts and guides to find on how to exactly do this and how best to do it.

So let me start:
I am currently looking to build a world. But in this world, I want to fully be able to create the landscape; meld, mold, warp, build, crash, flatten and much other else in those terms. But first I am dealing with quite the issue of one major thing.
It is not that the editor tools are bad, or my issue is with the development of an environment from using the editor tools. It is that I cannot create a way to set the parameters to have an extremely large map.
The size I am looking for, based on my calculations, and this is an over estimation for a large piece of land to cover everything I want, is roughly 118km by 118km. Although rather, I would prefer about 85km by 130km (or roughly around there.

Now the whole point of this post and this topic, is to ask; How do I create a map in Unreal 5 editor, in which I can pre-set the parameters to allow for an extremely sizeable open world map? And yes, the plan is to seriously look into making the lands by hand and not randomized. Even though this may take some time, It is not without reason for.

Lastly, were I to create this sizeable piece of land, can I integrate a coastline with the ocean water asset, but not make it an island, but more of a body of land against the ocean? In other words, say like I wanted to create the California coast line from San Diego to Santa Barbara, but extend the land inwards all the way towards San Bernardino? Can I do this without making a large piece of land, a large island, based on the editor in Unreal 5 and not using a third party add-on?
To continue my example:
Test1a

Hopefully that works.

Anyways. If there is any help to this #help post, please do not hesitate to give me any information, as it would be so greatly appreciated!

And if I missed anything or need to relay any more information, just ask me!

Thank you,