I found this post on TW center, I'm traveling right now so I can't test it but I wanted to post it here as a reminder and in case anyone wanted to make such a modification (god knows I did).

All credit to Quintus Sertorius of Total War Center:

Open Save game parser or any other tool that can edit save games.

Then open up your savegame and navigate to the following:

CAMPAIGN_SAVEGAME -> CAMPAIGN_ENV -> CAMPAIGN_MODEL

Here there are two values set at 20, the upper one is for land battles and the lower one is for sea battles. There are of course no sea battles in warhammer so you only have to edit the upper one.

Change both values to your liking, mine are set at 40, but it has been tested at 80 units per army.

Now save your savegame and load it up! You can now place more units in an army and the AI will do so also!

The only downside is that you can't actually see the extra units, you you'll have to write everything down on a paper or something. You can also count the small yellow blocks next to the banner. I'm using 40 units per army. You can always edit it so you can field more units, but you can never go down.

RIP my FPS but 40 unit stacks here I come.