Bear in mind Rojava, which operates at the commune level, has a population of 4.6 million people of different ethnic and religious backgrounds, with no known major internal conflicts.
couldnt you just say modern nation states are just “communities” of millions.
Modern nation states have top-down governments, which allows for corruption which is very difficult to eliminate. The Federated Anarchist model has bottom-up systems of governance, where power is far more distributed and thus far more difficult for any corruption to be wide-reaching, and far easier to eliminate.
Who decides when a ‘community’ is to big amd how it should be divided.
The community itself can decide that. If the constituents feel it's getting too big, they can form a separate community council that still confers and federates with the original community.
What happens when a community gets so big they either have to implememt population controls
They don't need to implement population controls, they just create more councils that all federate with each other.
or take neighboring community lands.
In a society of mutual aid, there is no real incentive to take other land. Any excess food or resources can be freely given to their neighbors who need it, as they can expect the same treatment in return. Mutual aid creates interdependent connections that reinforce good-will and cooperation.
Weve already been through this. For thousands of years we lived like this and barely survived.
None of those societies from thousands of years ago built inter-dependant federations of mutual aid to eliminate resource scarcity, they were top-down monarchies with kings who could arbitrarily declare war over any old thing like ego, resources, maintaining power, etc.
The times horizontally structured societies were tried in recent history, none of them were destroyed by internal conflict, they were always instead targeted and destroyed by external states with centralized exploitative power structures. Only Rojava and the Zapatistas survive, and just like before, Rojava is under extreme threat from Turkey and the new Syrian regime, both of which are centralized state governments who are imposing that force on others without public support from their populace, which is what top-down governments allow.
The moment 2 or more communities decide to work together and impose their will on the lands around them its over for everyone else, whatever the motivation.
There would be very little incentive to do that, but if it did happen, the other communities around them who are under threat of this could band together with all of the other communities they federate with in self-defense, similar to how the northern states banded together against the southern slave owning states in the US Civil war.
Simply put, its not scalable and will collapse as soon as any community get larger and more hungry than the others.
This shows a fundamental lack of knowledge of how Anarchism functions and avoids those issues. You speak very authoritatively on this subject for someone who I can only suspect has done very little research on it.