What happens when they don't know? Don't care? Don't want to know or care, because that would add tons of research? A common way out is to have you, a nameless strongman, simplify the bookkeeping by building a Space-Filling Empire.
So you want to build a Space-Filling Empire?Edit
Here's how it's done.
#1: March to the sea. If your Empire straddles sensible geographic borders and divides large ethnicities unknown to the writer, keep going. If your soldiers have to patrol the Sahara, the Congo Basin and every island in Micronesia, keep going. If you haven't swallowed an entire continent, or the writer hasn't told you to stop yet, keep going. Your empire will have hundreds of peoples with no common language, culture, religion or economy to unite them. That's okay. The writer isn't going to worry about the consequences; why should you?
#2: Establish a unitary state. Space-Filling Empire is an aesthetic. A very minimalist aesthetic. Ideally you should have no internal lines at all. There's nothing saying that you can't have them, but the author will be mad that they had to waste time filling them in. Do not anger your author.
#3: Close your borders and expel your journalists. So you've built an empire bigger than Alexander's. What are you going to do with it?
Nothing, of course! SFEs are the black holes of history: they're there, they're massive, but nobody goes into them and nothing comes out. Like children, mimes and Renaissance portraits, you're supposed to be seen and not heard. (Since the author will probably hint at some unspecified unpleasantries going on inside your single massive border, present-day Myanmar might be a good source of inspiration.)
#5: Decline! Sit on the golden throne of the Space-Filling Empire, and know that you have one of the largest countries on Earth. Sit on your throne and know that it doesn't matter. What really matters to the writer is a bunch of breakaway cotton plantations in the New World, or a mustachioed Austrian demagogue, or... anybody on Earth that isn't you, really, and anywhere on Earth that isn't yours. All your gold, your oil, and your teeming hordes of miraculously united people are powerless to change that.
Sit on that throne and weep.