The Rules Compendium says, "The point of origin of a spell is always a grid intersection," then later says, "An emanation spell functions like a burst spell, except that the magic continues to radiate from the point of origin for the duration of the spell" (135 and link mine).
However, many area spells that are also emanations see their areas centered on the caster. For example, the ever-popular 6th-level Sor/Wiz spell antimagic field [abjur] (Player's Handbook 200) has the entry Area: 10-ft.-radius emanation, centered on you.
Does the caster of a spell that has an area entry of emanation centered on the caster pick a grid intersection adjacent to his space as the spell's point of origin? If so, then can a big caster sometimes see that emanation affect less than the space the caster occupies? Or does the caster of a spell that has an area entry of emanation centered on the caster count squares from the edges of his space into squares around the caster… in a manner that's largely contrary to the rest of the game and the description of area spells themselves? Or is there another option that I'm just not seeing?
If addressing examples makes an answer clearer, I'm particularly interested in what area is covered by the antimagic field spell's effect when the spell's cast by a Gargantuan creature (that has a space of 20 ft.) and if that big caster suffers (or doesn't suffer) the field's effect. I'm also interested in if a living Gargantuan caster of the 6th-level Clr spell antilife shell [abjur] (PH 199–200) sees that spell effect prevent living creatures from approaching only some—rather than all—of the caster's squares.
Note: I cite the Rules Compendium because it's often considered the last word on the rules, yet here seems to provides little guidance, essentially parroting the Player's Handbook on Area (175). Feel free to use either source. I've not tagged this question rules-as-written because I don't really need this lawyered but playable, but rules supporting opinions are, of course, preferred. Inspired finally to ask by this question, but this's been bugging me for ages.