let me give examples, that's all I can start with. Also, run through the BofM and you'll see some numbers to start working with. Medieval army sizes aren't significantly bigger than classical/ancient Civ, so we should be ok.
Egypt had a standing army, (lots of extra food because of the nile, so they could support a class of people that didn't contribute to the subsistance) and the biggest Force I can think of that they every fielded was 20,000. They did, however, once fight a Hittite force twice that size.
Basically, tens of thousands, is probably typical. Something truly epic may reach 100,000, but that would be unique in history.
Remember also that most lands did NOT have a standing army, and "soldiers" were just anyone who needed a way to get by or were impressed into service. In Feudal societies you had the well-equipped wealthy nobles (knights) who were required to give a certain of time on campaign each year, and they would be responsible for bringing their own (peasant) foot soldiers.