When does a space empire become too large to govern tightly?

Is there an upper size limit beyond which a space empire becomes too large for its government system?

The limit depends on communications and delegated authority, but large empires would need federalized planetary and probably sector-level government.

How far a space empire can grow depends partly on the government it adopts. Tight central control places different demands on local administration from a system that delegates substantial authority. Planetary governments are proposed as the minimum level of a federal arrangement. Sectors or larger groupings could add further layers as the empire expands.