[RPG] Are there any guidelines in place governing the extent to which magic items can resize to different-sized users

Magic items in general don't resize, no. Clothing and jewelry are the exceptions (as stated in the text you quote), simply as a matter of game convenience.

Rule interpretation in general

There are two points to remember when looking at an ambiguous rule:

1. Your group, with the DM as final arbiter, decides the ruling for the rule. So, you need to establish what this is for each group that you play with.

2. Specific beats general. Start with the most general rule you can find and see how the specific rules change this.

Armor resizing in particular

For your issue, start with PHB p. 145 (ignoring the optional part because it makes no difference to the case at hand)

Variant: Equipment Sizes

In most campaigns, you can use or wear any equipment that you find on your adventures, within the bounds of common sense. For example, a burly half-orc won't fit in a halfling’s leather armor, and a gnome would be swallowed up in a cloud giant’s elegant robe.

There's your general rule: no armor that fits a PC race (size S or M) would fit a bear (size L).

Wild Shape (p. 67 PHB) says:

Worn equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of equipment, based on the creature’s shape and size. Your equipment doesn’t change size or shape to match the new form.

No help there; the armor will be too small and the wrong shape. In addition, there is a ruling that has to be made right here: is it practical for a bear to use armor even if it is the right size and shape? My ruling would be yes but your results may differ.

Now from DMG, p. 140:

In most cases, a magic item that's meant to be worn can fit a creature regardless of its size or build. Many magic garments are made to be easily adjustable, or they magically adjust themselves to the wearer.

The ruling needed here is does the hide armor a) need easy adjustment or b) magically adjust itself. If b) then all is easy, if a) then the druid may need to allow it to drop to the floor and then get some assistance to put it on which would take 5 minutes (PHB, p. 146) which makes it pretty useless in combat.

Those are the issues; your group needs to make the ruling.