You may be thinking about the rule regarding clothing or jewelry...Huh, that's odd - I would have sworn there was a rule that magic items automatically resized for the user, but I can't find it now.
Also note the exception for armor and weapons, since they do not normally resize.SRD said:Size And Magic Items
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. Size should not keep characters of various kinds from using magic items.
There may be rare exceptions, especially with racial specific items.
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 other size (91-100).
DMG said: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.
You may be thinking about the rule regarding clothing or jewelry...