Medium slime seemed to have a large slime's hitbox. Combined with that, I set a block near the slime on fire, and waited for it to blunder into it, which it appeared to do, and appeared to catch fire. (Producing a rather intimidating giant slime-sized fire.) But AFAICS the slime didn't take any damage. So I seemed to have a slime with a giant hitbox determining its own attack radius, and whether it was visually affected by nearby hazards, but when it came to the slime actually suffering damage from those effects, its giant hitbox didn't seem to apply. Cheating slime!