So the maximum world size for multiplayer is 20km^2 and problems arise outside that area. What happens if a single landscape chunk is scaled to be bigger than that let’s say the landscape is 30km^2 but we limit the players to something like 15km^2 from the center. Is there going to be any sort of problems with this too?
That will work fine. The problem is not the landscape but the world size itself. The farther you go from the origin (0,0,0) the less precise location calculations because of the floating point issues. Also for a single player game you can use world composition which will eliminate 20km limit.