Ok first calm down, i just try to understand whant you whant.
Second, sorry if my answer is not long but my native language (like lot of people on the community) is not english so i try to do it the most easy to understand.
Make a Large world can mean a lot of thing, you can make it whit a lot of instance or just whit a big instance. World streamer dosent work whit Atavism cause the World Streamer will change the coordinate of you caratere to 0.0.0 (or something like that)
The only probleme whit a Large world is the cpu/gpu you gonna use for it, the best way to optimize that it just to do a Local unload but you gonna need to script server side to send position of other player to your player when it close to him to load it . (you see i make a big sentence and i feal it confusing).
They have a lot of way to optimise your game but remeber something you cant make a good game whitout programing even whit Atavism. You will need to learn at lest the basic of c#. And no the community will not give you all script you need to optimise your game cause all game it different and need different thing.
And for the information Script they take time to make and a lot of time when you make it for your hown game. If you need script and you dont know how to make it but you are good at other thing ask for trade, i make a lot of trade in the past here.
And one last thing, Atavism still young and when someone find something they post it so if no one call about a limit in seize it beacause nobody have find one. for now i work on a 60000x60000 map and i have 0 probleme (it only for a test) but maybe if 300 player online i can have it, idk and i will do the test but the best way to know it it the try by yourself.