From the "Combining Magical Effects" section of the Player's Handbook (p. 205) and the Basic Rules (p. 85):
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
This rule can lead to one of two different rulings:
- The DM could rule that, since Enlarge/Reduce is a single spell, the more recent casting of it (Reducto's) would take precedence, and Inflatoman would not only return to normal size but actually become Small.
- The DM could rule that Enlarge and Reduce — while the same spell — are considered separate for the purpose of combining effects, and rule that the two effects cancel out, resulting in a Medium-sized Reducto.
If Engorgo were to stop concentrating, then Inflatoman would either remain Small (option 1) or become Small (option 2). If Reducto were to stop concentrating, then Inflatoman would become Large in either case.
Thank you to @V2Blast for notifying me of the 2018 PHB errata on this!
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.
Best Answer
The spell is still affecting the druid, and wild-shaping does not end effects on the wild-shaper, therefore the druid is still under the effects of the enlarge spell while wild-shaped.
At this point you might conclude that the question is settled: they would be an extra-large bear, because the spell is still working that that's what enlarge does. But that's not quite right.
Let us look at precisely what the enlarge spell's effect is, not what we might be tempted to mentally simplify it as (with all emphasis mine):
Okay, so it doesn't make it 2× in every dimension as a static fact, it makes it grow at the beginning of the spell. That would mean it would not make any further changes afterwards, only sustain the initial change.
Well, maybe I'm reading too much into "to grow". Let's see what else it says it does:
So it does look like it's a one-time growth, based on the verbs and nouns used. In particular, if enlarge is used on a target inside a smaller space than the maximum possible size, and then the target moves out of that space, they don't grow further once there is more room — the growth at the beginning of the spell is all you get, nothing more later.
So, the conclusion there is that the druid can shapechange into a form, and the enlarge effect is not applied to the new form. If they change into a sparrow or a bear, they get the normal statistics of a normal sparrow or normal bear.
That means that a druid that wants to dodge an inconvenient enlarge or reduce can easily do so by wild-shaping, but a druid that wants to benefit from enlarge or reduce in their wild-shaped form will have to somehow arrange for the spell to be cast after they wild-shape, so that it will grow their new form instead of just their previous form.