I love the warfare system, but I can't wait for the release of the second book for expanded rules!

The only essential rule I need is what to do if the caster of the group wants to cast a big spell on a unit.

No, I don't want to tell them it doesn't have effect, that's lame, and Yes, they will probably be engaged with an encounter, using a big spell slot towards the battle could be wasteful!

My current idea is: if the spell does damage, and the area is big enough (let's say 20ft radius), roll a Spell Attack (Prof spellcasting mod.) vs the Unit's Toughness. if it succedes the unit loses Sizes equal to half the spell slot spent to cast the spell, rounded down.

So Grh'tuck the mage casts Fireball at a unit of Ghoul Regular Medium Infantry. The mage rolls 7 vs toughness 17. Success! The Ghouls loose 1 level of size! If it had been cast with a 4th level spell slot the Ghouls would have lost 2 levels of size!

What do you think? What are your rulings? If you prefer to ignore this type of casts, why?

I realize that a focussed team of casters could wipe out units very fast! But I think they should be able to!