If I am under the effect of a spell effect that extends in a radius centred on me and then my size is altered to the next larger size category or next smaller size category, is the spell effect radius similarly affected by my new size or does is remain unaltered?
The reality is that we just don’t know
I have searched high and low for solid rules about effects “centered on you,” and they simply do not exist. All we have is that phrase, and the default area-effect positioning rules, which conflict with one another since your “center” is (often) not a grid intersection as proscribed by the area-effect positioning rules.
Certainly, insisting that the lack of rules means we “must” fall back on the default area-positioning rules leads to very poor results. Beyond the simple contradiction between what the word “center” means, this can cause nonsensical results in many cases, where an aura, burst, or emanation can’t even reach out beyond your own space, if you are Large or larger. Those rules simply do not cover this situation, and pretending they do will not improve anyone’s game. So both “legalistically,” based on the contradictory terms used by the limited rules we have, as well as practically, this is not and cannot be our answer.
But we have nothing else to go on, as far as the official rules are concerned. We have considered, and rejected, the only thing we have that goes beyond those three words.
Spell effects “centered on” a creature are measured from the edges of that creature’s space.
So yes, when you become larger, so does the area of the effect centered on you. This is fine; large sizes are usually negative for a spellcaster, and it’s not especially problematic to handle matters in this way. This is the ruling I have used in every game I have run, and it has never caused a problem. Alternative options, when other DMs have implemented them, have always caused problems.
- Though, of course, Paizo, being Paizo, had to confuse matters even more by making the ruling in an FAQ that was specific to certain sizes and never clarifying if the ruling should apply to all sizes—which would be consistent and appropriate—or if somehow the rules for spell areas changes with larger sizes, which is an inconsistent mess but maybe consistent with the unclear way they wrote the answer.
Per the SRD,
Regardless of the shape of the area, you select the point where the spell originates, but otherwise you don’t control which creatures or objects the spell affects. The point of origin of a spell is always a grid intersection. When determining whether a given creature is within the area of a spell, count out the distance from the point of origin in squares just as you do when moving a character or when determining the range for a ranged attack. The only difference is that instead of counting from the center of one square to the center of the next, you count from intersection to intersection. (https://www.d20srd.org/srd/magicOverview/spellDescriptions.htm#area)
So you always pick an intersection for it to emanate from, not just from "you" as an amorphous object. I would say it's fair that you can reselect that point when your shape/size changes.
The spell remains the same size, though its exact positioning might move slightly.
Lets say that you have Magic Circle Against Evil cast upon you, so it's a spell with a 10 foot radius that is centered upon you. Most DMs have the spell snap to the grid, so if you're medium, then the center of the spell is centered on a corner of your 5x5 foot/1 tile space. If you suddenly become large, say via Enlarge, most DMs will have it now be centered at the intersection of your new 10x10 foot/2x2 tile space, though some may let you center it on any intersection that your space is adjacent to. Since the spells center may move when you go up in size, the edge of the spell will move accordingly.
As a side note, some DM's will have you center the effect on your center, rather than on the of a corner of the square you occupy, though this is not recommended, as it makes the edge of the effect not line up with the grid as well.