I’m fairly certain I’ve mentioned this before, and alluded to it last session (that Mike’s character practically had to reach down to his knees to grab a bead off the necklace of fireballs), that I’m not a fan of items magically resizing themselves. So they don’t.
There are however methods to change their size magically. Essentially, an enchanter can put the magic item through a brief recreation, fulfilling any of the prerequisites of the original enchantments (proper item creation feat, spells, etc.). In addition, the is an additional (non-bypass-able) spell prerequisite of any spell that modifies the size or effective size of an item (such as shrink item, gravity bow or SHILLELAGH [’cause even in writing you have to scream it]). This process takes half time it would take to make the original enchantments and costs one tenth of the items market value (and thus cost 20% of market value to have an NPC do it for you), and the “re-enchanting” can change it to any size bigger or smaller that you want it to end up as.