How large is it possible for a PC to get?

Question

How large is it possible for a PC to get?

For clarity, here are my parameters:

1. This is about size categories, not 'feet' or 'inches' or 'meters' or any other precise measurement. This is about Medium to Large to Huge to Gargantuan categories.
2. This is for an NPC charlatan using this to trick people. (It's an NPC but will have a full PC sheet, so the distinction between PC and NPC doesn't matter).
3. Any means of size category enlargement works for this purpose, including magic spells, magic items, etc. Illusions do not count because it's about actually becoming larger, not just casting an illusion of a gargantuan giant version of yourself.
4. The character must remain the same physical structure. Polymorphing into a Gargantuan Dragon would not satisfy the need here.
5. Consumable items are fine.
6. This does not need to be permanent.

Answer 1

Gargantuan

The PC can become Gargantuan. There are a couple of methods to achieve this:

Method 1: Wish

Wish can do anything the GM allows, including temporarily or permanently increasing the size of a PC to Gargantuan.

Method 2: Potion of Giant Size + Enlarge

The Potion of Giant Size is a Legendary magic item from the Storm King's Thunder adventure:

When you drink this potion, you become Huge for 24 hours if you are Medium or smaller, otherwise the potion does nothing.

The PC can drink this potion, becoming Huge, then cast (or have someone else cast) the spell Enlarge/Reduce upon themselves:

You cause a creature or an object you can see within range to grow larger or smaller for the Duration. [...] Enlarge. The target's size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category - from Medium to Large, for example.

This will bring them up to Gargantuan (for one minute, the duration of Enlarge/Reduce).

Answer 2

It is possible to reach gargantuan size with the spell Wish and a permissive DM

The Wish spell states a number of primary specific effects/usages but also allows for an open ended effect:

You might be able to achieve something beyond the scope of the above examples. State your wish to the DM as precisely as possible. The DM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong.

Becoming Gargantuan might entail some pretty hefty setback, but the rule for Wish legally states that you can get any effect you want that your DM allows.