Question: Can a creature use spiked armor (PH 116 and 121 and cf. armor spikes on PH 123, 124) that is 1 or 2 size categories inappropriately sized (PH 113) so that the creature employs the spiked armor as a 1-handed or 2-handed weapon for use with the feat Power Attack (PH 98)?
Followup Question: If yes, does the inappropriately sized spiked armor occupy the creature's limb or limbs because it's now a 1-handed or 2-handed weapon? That is, can that creature who makes a full attack with the inappropriately sized spiked armor also use its natural attacks that are typically arm-based (e.g. claw, slam) as secondary natural attacks (i.e. at a -5 penalty and half Str bonus as a bonus to damage) or a light or 1-handed weapon as an offhand attack, suffering the standard two-weapon fighting penalties?
Context (or, "Why would this ever be a thing?")
The monster responsible for this question is a warden archon (BE 163-4). They're anthropomorphic bears with an extremely tame selection of spell-like abilities, fair DR that's overcome by their most common foes, spell resistance that crumples against foes that matter, an aura of menace that's based specifically on their class levels, and… um… scent, I guess. They can teleport but can't fly. They wear heavy armor yet have Tumble as a class skill. They don't have Balance, Intimidate, or Use Magic Device as class skills. They have no listed ranged attacks. They are a sad, sad CR 8, and totally vrock chow.
…And according to random Internet research Lunia, the first layer of Dungeons and Dragons' Seven Mounting Heavens of Celestia, is divided into 196 provinces, each under the command of one of these bear dudes.
I have some non-LG high-op 15th-level PCs arriving shortly on Lunia, and they'll encounter these CR 8s as the first line of defense, and the 100-Acre Wood rejects are toast if the PCs engage. I'm okay with that, but I want there to be some hesitation if the PCs decide to take on a province-ruling warden archon. Advancing the warden archon's HD to maximum (so he's Huge and Str 38 and CR 16) is a good start, but short of slapping class levels onto one (which I'd like to avoid because, y'know, Lawful Good should be good enough), the only thing I could come up with was exploiting the ridiculous BAB… except the bear dudes rely on their claws, and they don't meet the prerequisites for the feat Rapidstrike (Dr 73). I then considered the feat Blood-spiked Charger (PH2 92-93… shame about the errata), which is normally as lame as the warden archons themselves but sucks less when used by a Huge grizzly-ball of Power Attack-fueled spikes…
…but spiked armor is a light weapon. That means he'd use the Blood-spiked Charger feat's spiked avalanche maneuver to inflict the same amount of damage with his spiked armor that he'd inflict if he just used a two-handed weapon and the feat Power Attack, a prerequisite for the Blood-spiked Charger feat (albeit without the attack penalty), and he'd be better off (as is almost anyone) with–deep sigh–the feat Shock Trooper (CW 112), which, frankly, I'm a little tired of.
But then I thought, "Hey, can I just make the armor spikes bigger?" And then I thought, "I have no idea how that works."
Best Answer
You cannot wear armor that is not sized for you
Most items are allowed to work when missized, and weapons work with a penalty, but no such rule exists for armor, and armor is referred to as having a particular, fixed size.
Armor spikes don’t necessarily have to be the same size as the armor they’re on
There simply isn’t a rule anywhere that says so.
At first glance, this seems absurd and an oversight, but I think a case can be made for “extra large” spikes. This is most notable in the case of armor made for half-giants and goliaths, as their Powerful Build means they are somewhere halfway between Medium and Large, and can use Large weapons without penalty though they wear Medium armor. For a goliath, it makes sense that they would wear Medium armor with Large armor spikes.
The handedness of armor spikes is undefined
Armor spikes are supposed to be Light weapons, do not get used in any hands, and at no point do the rules address over-large spikes directly. In theory, Large armor spikes on a Medium creature would now be one-handed weapons and thus eligible for wielding “in two hands,” but there is no way to actually use two hands while wielding them. Thus, I do not think it is possible to get the 1½ Str to damage or the 2:1 Power Attack ratio with this method. They do get the Str to damage and 1:1 ratio of a one-handed weapon, because that is what they are. The rules do not actually say that one-handed weapons get those while “wielded in one hand,” unlike the two-handed versions.
But you could take things a step further: Huge armor spikes on a Medium creature are two-handed weapons. As with one-handed weapons in one hand, two-handed weapons don’t actually specify that you only get the improved damage when wielding them in two hands, because that’s assumed. The text is “two-handed weapons and one-handed weapons wielded in two hands.” So in this case you would get improved Str to damage and Power Attack. And a −8 penalty to attack for wielding a ludicrously oversized weapon.
The armor still does not take up any hands, in any of these cases.
This really does not seem like the right way to go
You have a monster that’s given a higher CR than it probably deserves, against PCs nearly twice that CR in the first place. The sensible thing to do here would be to houserule them into something better, or at the least give them class levels (I recommend Fighter 6 with the Dungeoncrasher ACF from Dungeonscape, or else Crusader or Warblade). This particular collision of the rules is wonky, and still will not provide particularly effective combatants.