Per the Wild Magic sorcerer's Wild Magic Surge table (PHB, p. 104)
07-08: You cast fireball as a 3rd-level spell centered on yourself.
There's other questionable area spells in that list (grease is a 10 foot square, so your square and which three?), but I'll focus on spheres like fireball (PHB p. 241):
(...) Each creature in a 20-foot-radius sphere centered on the point must (...)
My problem is, the sources I found so far - including the other answers to similar questions here - talk about measuring spheres from grid square intersections, whereas this clearly talks about it being centered on the unlucky Wild Magic sorcerer. Plus, since it's a wild magic effect, it doesn't exactly have a caster that can pick a point of origin. Is there an accepted way to measure an effect like this?