A couple of the online UE game use 30+km … Ark for example is reported to be 36km x 36km.
They are using World comp, and, multiplayer. Yes, they are using a edited 4.5.1 UE engine, but, i would think the results would be the same for supported mass landscape size in mutiplayer. Not saying your wrong, just saying that there are a few examples of this being achieved already (even on 4.5.1).
When doing new maps for Ark, the only thing i had to be aware of was keeping X and Y between 1,000,000 and -1,000,000 for replication reasons. (people and dinos would fall into the world and become invisible after those location points)