its much larger than that. It can probably be even bigger (certainly with wood floors instead of stone). I got 2 floors and plenty of space in this lofted outpost... could probably have done 10x10 floor tiles in stone or twice that in wood...
but 5x5 is plenty for an outpost.
https://steamcommunity.com/sharedfiles/filedetails/?id=2814526727

https://steamcommunity.com/sharedfiles/filedetails/?id=2814526713