Depending on How You Read the Enlarge/Reduce Spell, Go Wizard (or Bard) for Absolute Gimmicky Extremes
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.
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.
Wizard for Double Concentration Glyph of Warding Shenanigans
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.