This was true before the 20th century but my opinion is that instant communication, payment and relatively fast weapons delivery meant for over a century now th

This was true before the 20th century but my opinion is that instant communication, payment and relatively fast weapons delivery meant for over a century now this rule is no longer valid. Your empire can be as big as you want it to, even expand to the moon but probably not beyond. The limiting factor now is the appetite for war and conquest by a population that can communicate and organize efficiently to overthrow or change rulers as well as interference from global nuclear powers.