The best answer here might be a really unsatisfying one: it varies wildly. This shouldn't be too surprising though, given that conflicts today also vary wildly in scale. Its also hard to nail down specifics, because medieval chroniclers have this annoying habit of just guessing at numbers, contradicting themselves and each other, and generally tweaking around the numbers as they go.

Major conflicts, like the battle of Arsuf in the Third Crusade, might've had as many as 20 or 30 thousand men to a side. The 1169 Norman landings in Ireland had a total military force of under 1000.

To answer your question of the accuracy of fantasy novels/movies/ect., the tens of thousands certainly isn't impossible. The VAST majority of medieval battles, however, would've seen significantly smaller numbers. Remember that the Third Crusade was a massive military operation involving multiple European kingdoms. But then again, a lot of Fantasy novels and the like are set in the grand scale of determining the future of the entire world/continent/whatever, so it makes sense for them to draw inspiration from the biggest battles in Medieval history.