The DMG says:
"In most cases, a magic item that's meant to be worn can fit a creature regardless of size or build. Many magic garments are made to be easily adjustable, or they magically adjust themselves to the wearer."
"Rare exceptions exist. If the story suggests a good reason for an item to fit only creatures of a certain size or shape, you can rule that it doesn't adjust. For example, armor made by the drow might fit elves only. Dwarves might make items usable only by dwarf-sized and dwarf-shaped characters."
DMG, pg. 140.
Basically, the default is that stuff resizes or can be adjusted without having to explicitly state one is adjusting it. This is mostly for ease of play and DM'ing, but it's not a hard and fast rule; it's just guidance. The DMG leaves it up to you. However, you should be consistent in your rulings.
Personally, I use the variant rule from the PHB as to equipment sizes (though since this is in the PHB equipment section, I take this as implying it's specifically for mundane equipment). Even if something is made for a creature of the same size category, doesn't mean it fits. With clothes, that may only mean that they look ill-fitting. With armor, it may mean it has a penalty or imposes disadvantage unless modified by an armor smith.
For magic items, I rule that armor magically resizes - but only within its original size category or race that it was made for. In other words, human size armor won't resize to Halfling. Elven and Drow armor will only fit other Elves. Human sized armor adjusts to any medium sized Human. Etc.
I also rule that magic armor can't be modified by any-old armor smith. Doing so will destroy the magical capacity of the armor. It requires someone that can make magic armor in the first place, and likely requires substantial money or materials (magic components).
I don't do this to be mean or contrary. I do this to open up story possibilities and hooks, and to maintain a certain level of logic to the story environment.
But, this is entirely my personal preference when DM'ing. You can - and should - do what feels and works best for you.
P.S.: Some XP for you - to get rid of that little silver XP bar.
