Big Monsters and Antimagic

The spell Antimagic Field has a 10' radius area, centred on the caster. What happens when, say, a Gargantuan dragon (face 20' x 40') casts it?
Does it successfully cancel any enchantments on itself, removing its DR among other things, thus making its unaffected opponents stronger?

good question i would say that it resizes to fit him... at least that's how i would handle it i dont see any flexibility in the spell description.

At the moment I have two ideas, and I'm sure neither are by-the-rules.
1) The creature gains a 5' fringe around its body; this area (and the creature) are affected by the antimagic field.

2) The antimagic field is centred on the creatures head - it chooses on its turn where the head is, and the outside edge of that square is the centre of the effect. So half of the antimagic field is inside the creature, and a hemisphere is outside the creature.
This has some interesting effects - people behind it or further down the body are unaffected, but so are any attacks the creature itself makes to those squares. It can use the antimagic zone tactically (but of course can only select its area as a free action on its turn).
Obviously, it can't cast spells.

I don't know which approach I like best yet.