[Bruno]

Spells that can target parts of a thing are usually pretty clear on that.
Melee and Ranged spells give you a choice of hit locations and you roll against your melee attack skill or Innate Attack with the usual hit location penalties.
The various shape-changing spells only change part(s) of your body, and specify which part in the spell description. These kinds of spells are not flexible. If you have Tiger Paws, you don't turn entirely into a tiger. Likewise, learning Shapeshift (Tiger) doesn't let you change just your hands into tiger paws.

There is no Partial Body Of Fire, therefore you can't partly turn into fire. You could research that, but then you'd have to really clearly define what the heck the implications would be. There is no Partial Explode, and Explode doesn't have a to-hit roll against any skill, so affects the thing and the whole of the thing.