Does magic armor automatically resize?

Question

I tought I was pretty sure this is not the case, but some discussion has arisen and I'm not so sure anymore.

What I know:

• Most magical objects in the game auto-resize when worn by creatures of different sizes.
• There is a table detailing weight and cost of non-medium armor. This could still be useful for mundane armor, even if magic armor automatically resized
• Some spells explicitly tell us that armor does resize, as an exception to what happens normally. Again, this could be an exception for mundane armor only
• No distinction is made between a huge creature trying to wear a medium magic ring or a magic-ring-wearing human becoming huge by some polymorph spell. Might a similar distinction be in order for armors? (See my previous point)

I'll only accept the answer that quotes some manual (or FAQs, but only if no manual solves the problem) on the matter.

Answer 1

Armor and Weapons do not resize

Rules Compendium (p 84-85) says it more explicitly than the core rules do (emphasis mine):

As long as you’re the same size category and the same general shape as the armor’s original owner, the armor functions normally for you. However, inappropriately sized or shaped armor can’t be worn. Armor doesn’t resize to fit a wearer of a different size category, nor does armor constructed for a humanoid-shaped creature fit a nonhumanoid-shaped creature. In cases where a nonhumanoid-shaped creature tries to wear armor created for another nonhumanoid, the DM should use his best judgment.

Shields don’t change size to match the wielder. You can’t use an inappropriately sized shield.

Weapons don’t change size to match the wielder. You can wield an inappropriately sized weapon with a penalty (see Inappropriately Sized Weapons, page 151). Regardless of a weapon’s size, as long as you can hold a weapon you can activate its magical abilities.

Answer 2

In Dungeons and Dragons 3.5

Magic Armor and Weapons Don't Resize

The Dungeon Master's Guide (2013) from the section Size and Magic Items on page 213 reads

When an article of magic clothing or jewelry is discovered, most of the time size shouldn’t be an issue. Many magic garments are made to be easily adjustable, or they adjust themselves magically to the wearer. As a rule, size should not keep overweight characters, characters of various genders, or characters of various kinds from using magic items. Players shouldn’t be penalized for choosing a halfling character or deciding that their character is especially tall.

Only say "It doesn’t fit" if there’s a good reason. Cloaks made specifically by the selfish, self-absorbed drow elves might fit only elves. Dwarves might make items usable only by dwarf-sized and dwarf-shaped characters to keep their items from being used against them. Such items should be the exceptions, however, not the rule.

Armor and Weapon Sizes: Armor and weapons that are found at random have a 30% chance of being Small (01–30), a 60% chance of being Medium (31–90), and a 10% chance of being any size of the DM’s choice (91–100).

Therefore magic clothing (which is not the same thing as magic armor) and jewelry resize, and, specifically, magic cloaks should resize. Other items don't resize.

But a case can be made for ambiguity allowing armor to resize despite the final note if the reader conflates magic clothing, magic garments, and armor. (Seriously, as Tridus's answer explains, it took the Rules Compendium to come out and say exactly what was going on in no uncertain terms--and the details there about magic shields were entirely new.) However, such a reading of the core rules permits the system to be gamed using the chart Armor for Unusual Creatures (PH 123). Employing this misreading, a clever magical armor merchant can craft or buy mundane Tiny or smaller armor at reduced cost, make the armor magical, then let the buyer resize the armor to fit. I'm certain war profiteering from grig-sized chainmail--while amusing--wasn't intended.

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Armor Sizes in Previous Editions

I thought this confusion might've arisen from the rules in previous editions, but armor only ever resized in Dungeons and Dragons, 3rd Edition (see below). This must be an extremely common house rule (or misconception) in many games because the number of forum questions that show up about this topic via a quick Google search is impressive.

Advanced Dungeons and Dragons
The Dungeon Master's Guide (1979) says that "65% of all armor is man-sized, 20% is elf-sized, 10% is dwarf-sized, and but 5% is gnome or halfling sized" (124).

Advanced Dungeons and Dragons, 2nd Edition
The Encyclopedia Magica says, "When adding magical armor to the game, be aware of sizing problems: 65% of all armor (except elven chain mail) is human size, 20% is elf size, 10% is dwarf size, and only 5% is sized for gnomes and halflings" (67).

Dungeons and Dragons, 3rd Edition
The Dungeon Master's Guide (2000) says, "When an article of magic clothing, jewelry, or armor is discovered, most of the time size shouldn't be an issue" (176). Emphasis mine.

Answer 3

First, magic armor doesn't resize. But there was a special weapon quality "sizing" that cost a plus 1 bonus that let them do it, and I think armor should be able to do the same.

Also, don't be a slave to the treasure tables. Sure you can random roll out every item in detail if you feel you must; but if you came up with some studded Leather armor, make sure it fits the rogue or whatever. Just cause the DM runs the bad guys doesn't mean he has to be an ass about it, ya know? Now I know I just stepped on by guygax' s toes there, but we don't random roll out the dungeon layout or just use random encounters for everything. Gets stupid quick. Sure, non-intelligent monsters that can't use treasure can have all the random swag ya want, but intelligent creatures might just know of a kobold magic shop in the next mountain range. The PC's do it all the time, so intelligent monsters should too. Just sayin