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:

• Be from any race.


• Be from any class.


• Use any magical-item combination.


• Use temporary buffs/magic, but only if can use it himself. (Don't need another player/NPC)


• Be level 20.


• Size changes can be in any form. Does not have to maintain original shape/race/type.

The more vanilla, the better.

As a Moon Druid: all of 'em from Tiny to Gargantuan

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.

Get Gargantuan

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.

Get Tiny

You've got a lot more options here with your Wild Shape and can pretty much pick any from this list of tiny creatures.

Shapechange your way to it

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.

A Druid 17/Wizard 3 will give you a lot of options

The key spells/features are:

• Shapechange: a 9-th 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 2-nd level Wizard spell which lets you put your size up or down one category (Concentration, 1 minute)




• Polymorph: a 4-th 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.

A range of Tiny to Gargantuan in two forms

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).

• Tiny through Medium.
  
  
  • This is achieved by the Baboon form which starts as small
  
  
  • 
    Enlarge/reduce cast at-will from the staff of the magi opens up the options of tiny and medium.
  
  


• Large through Gargantuan
  
  
  • This is achieved by the Giant Ape form which starts as huge
  
  
  • 
    Enlarge/reduce cast at-will from the staff of the magi opens up the options of large and gargantuan.
  
  

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...

• Small through Large
  
  
  • This is achieved by any medium race in your normal druid form.
  
  
  • 
    Enlarge/reduce cast at-will from the staff of the magi opens up the options of small and large.
  
  

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