There is nothing in the rules regarding resizing magic weapons.
However, if you want to borrow from another principle, the peicemeal armor rules (Ultimate Combat p199), only the major portion of an item (typically the torso peice) is carrying the magic. So, resizing a spear (the haft only) should not affect the magic of the weapon.
Alternately, you can provide a magical solution similar to the spell Shrink Item. (Allow it to affect magic items and have it change it only one size.)
Skeld, it is not a house rule that magical equipment (other than weapons and armor) resizes to fit the wearer.
Size and Magic ItemsWhen 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).