[RPG] Does the Enlarge spell stack if it is cast multiple times

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:

1. 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.
2. 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!

No.

From the polymorph subschool:

You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell. In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph spell.

While the above rule is enough for your case, there is also a Paizo FAQ which says that size changes never stack, even for spells that lack a non-stacking clause:

As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies.

However, I can't actually find the "rules on size changes" that the FAQ refers to. So afaict, like many FAQ entries, this is an errata and not just a clarification of previously existing rules.