So … here is the “catch”
You cna make it as big as you want, but, thats a big misleading.
You have to stay within X/Y 1,000,000 / - 1,000,000
Actual is like 1.2mil/-1.2mil
Issue is that replication and the engines (at the version of ARK) ability to manage things the further from X/Y 0 you get.
A good sign things are about to go wrong: dinos fall into the landscape and become invisible but can still attack and be attacked.
As @duke54 stated, foliage culling is key.