This came up once in our epic game. We were fighting several collossal blue dragons. After ripping them new ones, about half managed to flee to saftey by force-caging me (it took me a round to get out with a miracled disintigrate) and flying at top speed.
Later they came back for us, surrounded by anti-magic fields. Immediately, the wizard said that they could not possibly be covered by anti-magic field, as the spell is only a 10' area, and they were each much, much larger than this. The DM stated that they had each cast it multiple times.
That worked out just fine for us, because this meant that there were multiple chances per dragon that the wizard's Disjunction would cause one of the fields to fade away, giving us our opening. Unfortunately, we ended the session at that point, before the disjunction was cast, and we never picked up that fight again. Fooey.