I considered writing a long post on the logical (im)possibilities of living fire that doesn't need fuel or oxygen here, then I remembered magic.
Small Fire Elemental
These guys are pretty cool (ironic, I know). The net loss to stats hurts, but two free feats are always welcome and their slams have a nice rider effect. 50 feet is as good as it gets for small creatures, and the fire subtype will probably be a boon overall.
However, is all this worth 2 subpar RHD? A fairly typical melee character is still outdamaging them, and fire elementals lack much utility beyond setting stuff on fire. +0 LA is fine.
Medium Fire Elemental
More fire damage! Also stat boosts. I'd say +0 still works here (with -0 looming ever closer).
Large Fire Elemental
Aaaaand RHD takes over. Back to the realm of -0 LA with ye!
Huge Fire Elemental
Elemental Purgatory aaaaaaaaaaw yeah! -0 LA.
Greater Fire Elemental
Homework for today, sketch the graph A = 2m where 'A' is my annoyance (measured in kilogroans) and 'm' is number of boring -0 LA monsters.
Elder Fire Elemental
Amusing observation: fire and air elementals are amongst the few creatures that get more dextrous when increasing in size. Not that it'll save them from -0 LA.