Just to keep things in perspective, the Roman Empire at its height could field 500,000 men. Keep in mind though that most of that 500,000 would be auxiliary intended for guarding and patrolling the empire's borders and not for campaigning. Medieval kingdoms were much much smaller for the obvious reasons of supply, logistics, and a much smaller population. The exceptionally large ones could field 100,000 men, while mid-sized ones were expected around 50,000. Most kingdoms back then had forces roughly around 20,000 men, which was considered a large campaign. 5,000-10,000 is your typical army size, with 1,000-5,000 being a much smaller army. Numbers that are in the hundreds, such as 100, 250, or 500 are considered garrison forces.
For further reference on how to size your armies, I suggest you check this out:
http://www.writing-world.com/sf/hordes.shtml