My question was more about hard splits: what to do when you have 2 DC's and they lose connectivity between them?

The larger pool can still have majority, but the smaller pool can not. Does the smaller pool change membership (because it can't see the larger pool) and continues on it's own? What if the smaller pool has a cold start and has no idea about the larger pool?

My point is that there needs to be some consensus on pool size to decide what constitutes a majority vote.

Implementations can make reasonable assumptions and rules for this, but afaict it's not covered by the protocol itself.