Frogjolnir:
EFFECT: "Tiny Symbol", but is still considered a standard character for equipping purposes. When this character occupies water terrain, modify attack and defense +1.

Wasp, Heart of the Avengers:
(-3) PYM PARTICLE RELEASE: FREE: Choose Tiny Symbol, Starburst symbol, Giant symbol, or Colossal symbol. Wasp, Heart of the Avengers and adjacent friendly characters (that aren't vehicles) gain that symbol until your next turn.

Rulebook:
Anytime a character can use a new damage symbol, that symbol replaces its most recent damage symbol and it becomes that new size.

and:
You can only equip standard characters.

If an effect causes an equipped standard character to
become non-standard (such as becoming Tiny or Colossal),
they stay equipped, but can’t use the equipment’s EFFECT
ability.

My proposal:
WaspHeart makes a Colossal character, say Surtur, into a Standard size. Surtur equips with Frogjolnir, which makes him Tiny. I believe he would stay Tiny when Wasp's effect wears off. Thoughts?

The theory is sound, and I don't personally see why it doesn't work, but I really just came here to say this:

Hold me closer, tiny Surtur

It almost works the only thing that prevents it from working is that Surtur still remains a 2x2 base character and those are still not standard no matter what size they are

Ah, yes...I should have thought of that. It's the same reason why I can't equip one of my new favorite pieces. XMAS Madrox. Funny thing is that I've had to make that ruling because of pieces like him I don't know how many times, and I still screw it up...was more focused on the joke than the rules

AH, yes, this part:

I'm losing my touch I got something RIGHT

Could work with some colossals, though.

That is CORRECT

bwahaha That is fantastic

He could be your Tiny Surtur.

Now I want to do this just for the sheer ridiculousness! Plus this old Surter could use the +1 attack and +1 or 3 defense