Ummm...do you have a source for Green Scourge? I can't find it.
In any event I will attempt to answer questions without the full information.
Flame blade spell is written to assume a medium character, and it describes basically conjuring a scimitar made of flame that is 3ft long (sized for a medium character). Presumably a small character casting the spell gets a scimitar sized for their character, so it is likely not a full 3 ft long, but would still do the same damage because damage is set by the spell.
Presumably, if you were if smaller if would still scale down and still do the same damage. And you would have the same reach and threaten the same area as if you were wielding a scimitar of the appropriate size.
Despite the weapon being immaterial it never says that you don't need a hand to wield it as normal, so near as I can tell nothing removes that restriction. So no, you can't wield it in your mouth or in paws.
Flame blade functions as a scimitar. Weapon finesse does not work on scimitars. Dervish dance would work though to allow you to get dex to attack rolls. Since the blade does not add strength to damage, you cannot get add dex to damage either.
Claxon,
I disagree with your interpretation that caster size matters. Here is the text of the spell.
(Bolding relevant text)
Flame Blade
Source PRPG Core Rulebook pg. 283 (Amazon)
School evocation [fire]; Level druid 2, hunter 2, shaman 2
Casting
Casting Time 1 standard action
Components V, S, DF
Effect
Range 0 ft.
Effect sword-like beam
Duration 1 min./level
Saving Throw none; Spell Resistance yes
Description
A 3-foot-long, blazing beam of red-hot fire springs forth from your hand. You wield this blade-like beam as if it were a scimitar. Attacks with the flame blade are melee touch attacks. The blade deals 1d8 points of fire damage + 1 point per two caster levels (maximum +10). Since the blade is immaterial, your Strength modifier does not apply to the damage. A flame blade can ignite combustible materials such as parchment, straw, dry sticks, and cloth.
You are creating and wielding a beam, not a scimitar.
.... POSSIBLE FLUFF HERE ....
You wield it AS a scimitar, so if you are able to wield a scimitar in your mouth (other poster) you should be able to wield the beam as well. Penalties might apply. I suspect this allows weapon focus to be usable, and casters normally capable of the spell will have scimitar proficiency.
....Back to da crunch....
These are Melee Touch attacks so perhaps anyone capable of Melee touch can use it. One might quibble that this could allow tiny creatures to attack adjacent squares, allowing Threat & Flank, but this is hardly clear, and should likely function the same as any other melee touch attack for that caster.
As I said, I know how the spell is written, but keep in mind it has had the same text since at least 3rd edition.
The fluff of the spell describes a 3ft blade. But the truth is it doesn't matter. You wield it as though it were a scimitar, presumably sized appropriately for the caster.
There isn't anything about the spell that is size dependent, except presumably that the blade is sized for the caster.
Or do you really think a goliath druid using giant form II to turn into a huge size creature still gets a 3ft flame blade?
Yes, if a gargantuan dragon cast the spell it would be a 3 foot beam. It would also be foolish, since it's claws are MUCH more effective. It is a spell beam, your size, strength, etc. give you no more than they give to any other Melee touch spells. You get reach, you may get better hit chances, but why would you get more damage? Do you get size bonus to "Frostbite" damage?
I never said it got more damage.
Did you read my post?
I said the spell always deals the same damage, but the size and reach of the weapon is determined as though it were created as a scimitar appropriate to the size of the caster.
Presumably a small character casting the spell gets a scimitar sized for their character, so it is likely not a full 3 ft long, but would still do the same damage because damage is set by the spell.
Sorry, somehow your multiple statements that damage does not change because of size did not stick in my mind. Changing the 3' beam to adjust for caster size really has no effect ruleswise that I can think of, since I really haven't looked into how size effects Melee touch attacks. I suppose it is a matter of taste if you want or don't want 9" sprites swinging around yard long flaming beams. Since I enjoyed the visuals of Yoda dueling with a full size lightsaber, you can guess my preference here.
Size of the weapon for the most part doesn't affect things for this specific instance. Since weapon damage is set, it could really only affect reach. And the fluff (size) of the weapon.
Wielding a large size weapon doesn't increase a wielders reach, and technically wielding a undersized weapon doesn't decrease a weapons reach.