Just for fun, I want to create a character who can vary from really tiny or smaller to huge or bigger.
Which is the bigger/smaller combination to create that "Ant Man" style char?
It can:
The more vanilla, the better.
You can cover all the sizes, and do so utilizing your own statistics by being a Circle of the Moon druid.
This works by activating your wild shape, which provides that:
Your game statistics are replaced by the statistics of the beast, but you retain your alignment, personality, and Intelligence, Wisdom, and Charisma scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature. If the creature has the same proficiency as you and the bonus in its stat block is higher than yours, use the creature’s bonus instead of yours.
Unlike with polymorph, Wild Shape lets you be you...but in a different beast form.
At 20th level, a moon druid can transform into up to a CR6 beast. The one to pick a CR5 Brontosaurus(nb: you will have to have seen one of these to do this), which is a gargantuan creature.
You've got a lot more options here with your Wild Shape and can pretty much pick any from this list of tiny creatures.
The 9th level spell shapechange while at level 20 gives you full access to up to CR20 creatures, many of which cover Gargantuan and Tiny sizes.
You assume the form of a different creature for the duration. The new form can be of any creature with a challenge rating equal to your level or lower.
The key spells/features are:
Shapechange: a 9th-level Druid spell which lets you change your shape to any creature, up to CR 20, as an action. (Concentration, 1 hour)
Enlarge/reduce: a 2nd-level Wizard spell which lets you put your size up or down one category (Concentration, 1 minute)
Polymorph: a 4th-level spell on both spell lists (we would know it as a Druid spell though) which lets you take the form of any beast (Technically up to CR 20, but the list stops at 8). (Concentration, 1 hour)
Wild Shape: which gives you two uses to be a beast up to CR 5 (assuming circle of the Moon)
Notably these spells and features stack poorly, either requiring concentration or Wild Shape preventing you from casting spells while transformed, but this will provide you the opportunity to transform many times (up to 1 shapechange, 11 polymorphs, 6 enlarge/reduce per long rest and two Wild Shapes per short).
Having the legendary Staff of the Magi would add unlimited casting of Enlarge/Reduce. (Thanks NautArch)
What race you are doesn't matter much to this approach, but since you care about size you might want to be a Goblin for their Fury of the Small feature.
To achieve this range, you will need the druid's level 18 Beast Spells feature and Combat Wild Shape, at least 1 level in sorcerer, warlock, or wizard, and a staff of the magi.
The two forms you will use are Giant Ape and Baboon (as these are the forms of appropriate size that have opposable thumbs).
Since enlarge/reduce requires no charges from the staff of the magi, you can switch sizes within the size ranges for each form as many times as you want within the maximum of 9 hours in Wild Shape. You can also switch size ranges once between short rests, making this character's size switching even more flexible.
For even more flexibility, you can also have access to...
Side note: As a high level druid, you would also have access to shapechange which offers a wide range of more powerful options for a short time in every size category
This class has the Diminution Psionic Discipline and the Giant Growth Discipline.
Diminution gives you access to:
Miniature Form (2 psi; conc., 10 min.). As a bonus action, you become Tiny until your concentration ends. While this size, you gain a +5 bonus to Dexterity (Stealth) checks and can move through gaps up to 6 inches across without squeezing.
Microscopic Form (7 psi; conc., 10 min.). As a bonus action, you become smaller than Tiny until your concentration ends. While this size, you gain a +10 bonus to Dexterity (Stealth) checks and a +5 bonus to AC, you can move through gaps up to 1 inch across without squeezing, and you can’t make weapon attacks.
Giant Growth gives you:
Ogre Form (2 psi; conc., 1 min.). As a bonus action, you gain 10 temporary hit points. In addition, until your concentration ends, your melee weapon attacks deal an extra 1d4 bludgeoning damage on a hit, and your reach increases by 5 feet. If you’re smaller than Large, you also become Large for the duration.
Giant Form (7 psi; conc., 1 min.). As a bonus action, you gain 30 temporary hit points. In addition, until your concentration ends, your melee weapon attacks deal an extra 2d6 bludgeoning damage on a hit, and your reach increases by 10 feet. If you’re smaller than Huge, you also become Huge for the duration
You can spend 7 psi on a discipline starting at level 9. If you multiclass at least 3 levels into Wizard or Sorcerer (Wizard is probably better, since it's also INT-based, like the Mystic) or if you're a Duerger, you also get access to Enlarge/Reduce, although you can't stack this with your Mystic abilities, since both features require concentration.
To become small, you actually need to cast Enlarge/Reduce (Mystic can't do that), and to become gargantuan, you would need someone to cast Enlarge/Reduce on you or get an item that gives the effect without concentration (although no official item does this, to my knowledge).
Do note that this class, while from an official source, is playtest material and therefore requires special permission from your DM to use. Regardless of your DM, it is not AL legal.
As many have mentioned the Enlarge Reduce Spell, which wizards can get at third level, doubles or halves a creatures size. It is unclear however, so far as I know, whether it can double the size of a gargantuan creature or halve the size of a tiny one. The ambiguous parts of the spell read:
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.
and
Reduce. The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal. This reduction decreases its size by one category - from Medium to Small, for example.
As you can see it has two direct size effects one of which (doubling/halving) has a straightforward game mechanic for any situation and the other of which (size category change) has no existing mechanic by which it can increase and decrease the size of a gargantuan or tiny creature respectively because there are no size categories beyond these. So essentially the issue is whether an element of a spell which, under casting circumstances, can not go into effect and has no provisions for this, nullifies the spell. This is open to interpretation, but if you are in the "it still works camp, proceed.
Assuming the enlarge reduce spell does work to further the extremes of a Gargantuan or Tiny creature, you then need a way to both be the creature and cast enlarge/reduce on yourself (on the assumption that you're not really being Antman if someone else does the casting). Shapeshift permits you to cast spells as such a creature, but you have a Shapeshift with Enlarge Reduce double concentration issue, since they are both concentration spells.
There are few ways to do this, but a wizard can do it provided one spell is a creature targeting spell level 3 or below, via the Glyph of Warding Spell which is available to both Bards and Wizards. The relevant parts:
Spell Glyph. You can store a prepared spell of 3rd level or lower by casting it as part of creating the glyph. The spell must target a single creature or an area. The spell being stored has no immediate effect. When the glyph is triggered, the stored spell is cast. If the spell has a target, it targets the triggering creature. [...] If the spell requires concentration, it lasts for the full duration.
Thus with an hour of casting Glyph of Warding you can make that Glyph stored Enlarge Reduce, then use that one level nine slot to Shapechange (or True Polymorph) into the extreme-sized creature you want, trigger the ward, and enjoy one minute of absolute size absurdity.
This would also be achievable with True Polymorph, and thus a Bard, who gets that and Glyph of Warding could take Enlarge Reduce as a Magical Secret and achieve the same effect. Choice of spells (especially Magical Secrets spells) is a much bigger decision for a bard though.
Note: Originally Glyph of Warding seemed to specify that it was only a hostile spell against other creatures. This has been changed in 2017 errata as discussed here.
One should also note that, even if sizes beyond tiny and gargantuan are impossible, one could use these tricks with a halfling wizard to go straight from halfling reduced to tiny size to giant enlarged to gargantuan size, which is about as Antman-ish as possible.
