How do I RAW get a Halfling from size Small to size Large (without using spells of levels 7–9)?

Question

How do I turn a size Small Halfling into a size Large combatant?

I am a GM and the PCs are Level 4. I want to throw the PCs a curve they are not expecting, while keeping it “mid-level” (not Mythic nor Epic). This is for an NPC.

The Enlarge Person spell says:

This spell causes instant growth of a humanoid creature, doubling its height and multiplying its weight by 8. This increase changes the creature's size category to the next larger one.

Enlarge Person also says:

Multiple magical effects that increase size do not stack.

The Polymorph spell says it functions like Alter Self. Alter Self says:

When you cast this spell, you can assume the form of any Small or Medium creature of the humanoid type.

Does that mean RAW my Halfling can somehow take a potion of Alter Self to become size M, then a potion of Enlarge Person to become size L? I suspect this is not intended to work.

While I would like the NPC to be a humanoid when Large, it could be another creature type.

What are the options to go up 2 sizes, RAW, without getting into spells of levels 7–9?

(I thought Duergar had this ability, but they start at size M, not size S.)

Answer 1

The spell giant form says that the spell's caster "can assume the form of any Large humanoid creature of the giant subtype." This will change even a Fine caster's size category to Large. However, even after this transformation, the general rule for size changing continues to apply. That is, the Pathfinder Core Rulebook FAQ includes, in part, this exchange:

How does damage work if I have various effects that change my actual size, my effective size, and my damage dice?

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.…

(N.b. There is, so far as I can tell, other than in the FAQ itself no general "rules on size changes" except in the description of transmutation subschool polymorph: "[O]ther spells that change your size have no effect on you while you are under the effects of a polymorph spell." Also see this question.)

Beyond this, a creature's size can be increased by 1 category instantaneously (i.e. forevermore nonmagically) by drawing from a Harrow Deck of Many Things, a minor artifact, the card the Mountain Man (i.e. the 5 of diamonds). (Good luck!) A halfling who drew the Mountain Man would be a Medium halfling normally and could use the spell enlarge person to become Large.

Answer 2

RAW there are ways to cast Mythic Spells without being Mythic per se, so:

• Mythic Enlarge Person https://www.aonprd.com/SpellDisplay.aspx?ItemName=enlarge%20person You can increase the target’s size by up to two size categories, to a maximum of Huge. is exactly what you're looking for.

Another way is to use:

• Possession https://aonprd.com/SpellDisplay.aspx?ItemName=possession, preferably on an ogre, giant, troll or whatever else large you're willing to give this NPC as a henchmen. It fits your requirement of not using spells of levels 7-9, because it's just level 5.

Depending on what your actual goal is (because you didn't specify it) illusions are also a thing. Be small, pretend to be large.