Unfortunately for dragons everywhere, Widen spell is +3 spell level and it only increases the area by 50%. That means only a 15 ft anti-magic field, still not big enough for the Great Wyrm.
However, since we are using info from splatbooks, consider Antimagic Aura (from Magic of Faerun) -- Sor/Wiz 7. It's like an Anti-magic Field except that it has a range of touch and it covers the whole target (Will negates). Clearly this spell was meant to be used offensively, but in the Great Wyrm's hands it would be deadly if used defensively.