Rule interpretation in general
There are two points to remember when looking at an ambiguous rule:
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.
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.
In the PHB pg 164 it gives you the multiclass table for what proficiencies you gain once you place 1 level into Druid (or any other class). Note on this table it also reminds you that Druids will NOT wear any armor or use any shields made of metal, just like it does in the Druid section earlier in the book. Thus your answer becomes: No, you may not wear your metallic armor you wore as JUST a ranger, and must remove it if you wish to become a Druid and use Druid powers.
It could be thought of like this: You were just a Ranger, and had no qualms about using metallic armor. You were curious to learn the ways of the Druid and have realized that before you can properly harness druidic magic and the abilities of Wild Shape, you must first take a vow to never wear metal on your person. Then the local Druids will teach you what you want to know.
Best Answer
It appears that they simply will not wear metal equipment. The PHB says they will not use metal equipment. It does not say that they are not proficient with it.
From page 65 of the PHB:
Since I can't find any mention of consequences for wearing metal equipment as a druid, and given the wording in the PHB, I can only assume that you have two mutually exclusive options: