With the exception of weapons, armor, and race-restricted items, the size of a magic item should not prevent its use by a character. (Emphasis mine)
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 race-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).
It explicitly states that jewelry and clothing resizes or is resizeable, and implies at the end that most magic items should be usable regardless of size. That said, magic staves don't function any differently if they are larger or smaller, so there's no reason they wouldn't resize to fit their wielder.
As a side note, some GMs rule that magic weapons and/or armor found also resizes to fit the wearer. As long as the rule isn't exploited (and it's a houserule, the GM can draw the line where ever they want), it saves characters a lot of time and resources they would otherwise be using to sell the armor that's perfect for them but doesn't fit.