The Enlarge/Reduce spell can make a creature one size category bigger.
Considering that two spell effects of the same nature cannot stack at the same place and time (you can't cast two Enlarges on the same Medium target to make it Huge), is there another way (spell, item, etc.) to increase a target's size category that could be stacked with the Enlarge effect in order to increase the target's size category twice ?
Per the Wild Magic sorcerer's Wild Magic Surge table (PHB, p. 104), if you roll the d100 and get a result of 93-94:
Your size increases by one size category for the next minute.
It doesn't specify that you cast enlarge/reduce on yourself. In other cases, it explicitly states that you "cast" spells:
You cast fireball as a 3rd-level spell centered on yourself.
You cast magic missile as a 5th-level spell.
and so on.
So enlarge/reduce the spell and the wild magic enlarge effect aren't spell effects of the same name and can be stacked.
Enlarge would stack with the spell polymorph or the druid's Wild Shape feature, which allow you to assume a different size category, based on the shape. If you want to remain the same creature type, I don't think you can.
Using a Potion of Growth might require a DM call, since it says it duplicates the effect of the enlarge/reduce spell, but is not actually the spell itself. I think most DMs would disallow the combination, but since the potion is a consumable, DM-controlled resource, it's unlikely to become unbalancing.
Level 14+ Giant Soul Sorcerers (Unearthed Arcana) can use Rage of Fallen Ostoria, letting them increase size category by 1 for a minute, once per short rest.
Starting at 14th level, you gain the ability to channel the souls of your ancestors into your physical form. When you start casting a sorcerer spell on your turn and expend a spell slot, youcan increase your size by one category—from Medium to Large, for example.
You can either cast Enlarge on top of that, or wait until level 18, when you can use it twice per short rest, and are explicitly permitted to stack it on itself.
In addition, you can now use Rage of Fallen Ostoria twice between rests, but no more than once on a turn. If you use that feature while under its effects, its increases to your size, hit points, reach, and walking speed are cumulative.
Monster Duergar have a once per short rest ability to gain similar effects to the enlarge part of the enlarge/reduce spell (it doubles weapon damage dice rather than adds 1d4 and requires no concentration, so usually superior). They become a large creature for the duration. Because this is a magical ability rather than a spell it can stack with the enlarge/reduce spell. And voila, you have a huge Dwarf.
The player race Duergar casts it as a spell, however, so unless you have a friendly NPC party member with the creature stats rather than player race stats there is probably not a time when you want to do this.
Just get the Rune Knight (from Tasha's Cauldron of Everything) subclass from the fighter and then use the Enlarge/Reduction Spell. Use Giant Might first and then Enlarge. Now you are a Huge Creature on Level 3 (when another Party moember casts Enlarge, if not you would have to get another class with the Enlarge Spell, so with no Help you can become a Huge Creature on Level 6). (If i understood it right). Or you could just use the Gray Dwarf subrace and get the Rune Knight subclass. You can now cast Enlarge on yourself and become a Huge Creature in one turn and on Level 18 you become a Gargantuan Creature.
How about a potion of giant size and then using enlarge/reduce spell as a stack. The first just states you gain the Huge trait; then using enlarge/reduce, you can therefore double the size of a creature.
Hummm... rune knight lvl 3 feral might... you become large. Huge at lvl 18. So with enlarge spell boom.
