Advice On a Seamless world

Okay, So I am Not experienced in creating games at all but I catch on very quickly, That being said I wont have a problem understanding any information provided
I really only Have one question SIZE! I want to make my own Seamless world about the size of the one shown in the video on youtube created by unreal
“Open World Kite Real-Time Demo in Unreal Engine 4” This is the link to the video
To better explain what I am asking help on Say I open a New Project and I were to Create a Terrain What Would The Size be?
Any Advice would be much apprecieated, Thanks - ThatBoy_v

The Kite Demo terrain is actually made up of smaller “chunks” of terrain from what I have seen.
Something you should do is install the Kite demo and take a look through it and it will help gain a better understanding.