Yeah, when x-reffing theoretical sizes for FE battles in cases where we have known armies, I tend to look at the Wars of the Roses, the Hundred Years' War, and various Italian campaigns-- things that straddle that late medieval/early Renaissance divide. The battle of Towton was an ungodly bloodbath, and that was 50K soldiers and 28,000 dead-- unheard of for the time. Whereas the strategically important battle of Wakefield a year earlier had fewer than 30K participants (maybe far fewer) and only about 3K dead, but it took out the Yorkist leadership on the scene. (Kill the Lord? Insta-win!)

And then in a couple of cases we outright don't have invisible armies alongside our PCs and we're given to understand that a unit is a unit-- FE4 in particular and I can't see FE5 being any other way either.