Armies of more than 500 soldiers are current in my game on Orania (I'm about turn 100 against mighty AIs). I think I have at least one or two big blob armies on my fronteers (or rampaging in my dominion) since 20 or so turns. The biggest I've seen was an army of 900+ from Ct'is, half lizards half undead and including about 50 chariots. My EA Ermor own a big third of the map but 2 of the 6 remaining AIs still have more troops than me.
Playing against 3 difficult level AI's, defensive, aggressive and standard. Commonly seeing armies of 150 - 300+ by 2nd yr. Still my 3 Agartha priest/mages, 40 blessed ancient ones can kick there butts both on land and sea. Finding it real hard to make headway however and the aggressive AI really is aggressive! Also Oceania (the aggressive AI) builds castles for fun!
The largest AI army what I've seen had ~1100 troops. However I only had 2 SP games so far on small maps.
Gandalf has mentioned that he has seen much bigger AI armies btw.
"Training is principally an act of faith. The athlete must believe in its efficacy: he must believe that through training he will become fitter and stronger, that by constant repetition of the same movements he will become more skillful."