dnd 3.5e – Size alteration while under a spell effect that is a radius centred on you

The spell remains the same size, though its exact positioning might move slightly.

The important thing to remember is that you shouldn’t be drawing the spells radius from the edge of your space, you should be drawing it from the center.

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. If you’re medium, then the center of the spell is centered on your 5×5 space. If you suddenly become large, say via Enlarge, then the spell is still centered on your center, but instead of being centered in the middle of a singe square, it’s centered at the intersection of 4 squares. Since your, and by extension the spells, center have moved from the center of a square to the intersection between squares, the edge of the spell will move accordingly.

As a side note, some DM’s will have you center the effect on your choice of a corner of the square you occupy if you’re medium or smaller.