They do have a relative size, which is a bit ambiguous whether it refers to cross sectional area or volume. But either way, size of body parts is a thing, yes.
However, a few issues:
1) The way you described it would imply something along the lines of an arm with a hand and fingers and everything being on fire for an hour and ending up as a tiny doll-sized arm that still has a working hand and fingers and everything... which needless to say, is not any more realistic. Fire doesn't shrink things, it consumes them outside-in. A more meaningful version of what you're getting at might be something more along the lines of "disable and destroy things in time and some-fire-resistance-based order of size, smallest first"
2) A limb has a bunch of different tissue layers. Treating the whole limb as one object in your suggestion ignores the possibility of something like a creature that has gasoline for blood but asbestos for skin. In other words, many types of tissues need to be able to completely burn away before other ones even begin to burn. More commonly, bone would be an issue.
3) Currently, fire in general is a little crudely specified, and you should probably expand your suggestion to include all types of fire. All that fire is right now is just "change a 0 to a 1 somewhere saying 'it's on fire', and raise the temperature to 200 degrees higher and keep it there as long as it's on fire, the end." There's no conception at all of fuel or oxygen or whatever. If you fix fire in general, you might fix creature fire for free, and the game would be better overall. I can offer suggestions, but there's lots of ways to maybe do this. Damage could be treated like "amount of fuel" for instance xwoodx is less burnt than XXwoodXX, blah blah. For body tissues, the color coding damage system serves the same role? But then how do you model things that hold little chemical energy versus lots? Dunno...
4) If the game is changed to make creature fire more realistic, especially if it's more devastating than before like you're suggesting, it would be nice to also have other changes at the same time to make creature AI better at putting themselves out or whatnot, to balance it.