Are there any guidelines in place governing the extent to which magic items can resize to different-sized users?
The description given under "Wearing and Wielding Items" (DMG, p. 140) 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.
Going by this, it's possible that upon the party defeating a fire giant wielding a magical greatsword, the halfling in the party could go touch the massive weapon, and it would shrink down to the size of a regular sword within seconds, given that there are no stated limits. I know that a DM can decide the extent to which this can happen to fit his/her own game, but is there any standard or given 'Rules as Intended' governing how this should be handled?