Well, you can have battles of about that size if you really want. Simply play byzantium in early, and have 20,000 or more men waiting for the horde to arrive. The size of stacks in the provinces they arrive in has some effect on the size of the armies, so without bringing in more they almost always have enough for a win on auto-calc. If you brought 20,000 crack troops that means they would have around 30,000.

I actually fought a few battles of that scale. On auto calc over 10,000 of mine and over 20,000 of theirs died. When I fought it we stood our ground for over 5 hours, re-inforcing when we had a breather and positioned close to the back of the map to allow for quick replacements. After all that time, over 5,000 captives (and the mongols really do not run away easily as anyone will know) and more than twice that number dead they managed to rout all my units on the field, and the effect was such that any troops coming on would run at once because of so many of mine routing.

A terrible failure, even though it was a fortress over 8,000 captives had to be ransomed (nearly took half my money, and this was the byzantines...) and then they made the mistake of attacking the fiortress, THAT was a slaughter.

THe battle took over 6 hours in all, and only ended because I saw it was hopeless and ordered all troops to flee. It was not fun past the second hour.