How big can empires be in spelljammer's.

The title pretty much says it all. I'm currently planning a spelljammer's campaign and I'm having trouble to set the size of some empires. For example in the Astral Adventure's Guide it says that githyanki have outposts in the astral sea but can empires also extend over multiple wildspaces? Maybe I missed something in the book but rn I don't find anything specific.