Could a Huge or bigger creature use antimagic field?
The antimagic field spell description says:
Range: Self (10-foot radius sphere)
A 10-foot-radius invisible sphere of antimagic surrounds you. [...] Until the spell ends, the sphere moves with you, centered on you.
The Variant: Dragons as Innate Spellcasters box under the Dragons entry in the MM (p. 86) allows a Dragon to cast spells of a level 1/3 their CR:
Dragons are innately magical creatures that can master a few spells as they age, using this variant.
A young or older dragon can innately cast a number of spells equal to its Charisma modifier. Each [...] spell's level can be no higher than one-third the dragon's challenge rating (rounded down).
So a CR24 Ancient Dragon can cast 8th-level spells and be Gargantuan.
A Gargantuan dragon's "self" is larger than 10 feet. So is there a ten-foot-radius sphere that pops up deep inside the dragon's body? Or does it surround the dragon's body to an area of 10 feet around it?
I'd tend to say the latter, but I'm not sure how to adjudicate that. The two parts of the spell description are mutually exclusive and neither is more or less specific than the other.