I know this question is going to come up today and want to have it clarified before we play today. So do Colossals take knockback?

They have Great Size, so no they can't be knocked back.

Unless you have thanos because he always knocks galactus on his a$#...
jk no they don't take knockback

And I cannot think of ANY way to make them take knockback, at present. There's no way to outwit the symbol, or change THEIR symbol.

Earthbound changes all combat symbols to the standard , , and . So, hitting a colossal with the with Swingline would change them. But that still wouldn't really matter, since most colossal figures are multi-based (Groot on 2 clicks, excluded) and still wouldn't take knock back.

Swingline on flying colossals gives them Earthbound but, and it is a big but *, multi-base characters can’t be knocked back (so what they are no longer colossal, they still have a 3x6 base!)

* I like big buts, I cannot lie

And now I know why I couldn't think of any: we don't use feats!

Also, the sparkly new Stranger can be colossal...

Hulkmariner can also cause... shrinkage.

TOTALLY different topic...oh, wait. You mean his SP that can cause earthbound, right?

Sowwy! My mind wandered...

Okay, so to sum up, colossals generally don't suffer knockback, nor do multi-square based units.
A character feated with Swingline making an attack against the Stranger could cause him to have the symbol due to earthboundedness. HulkMariner's SP can cause this to Stranger AND Groot for as long as H-M is on the map (!), when Groot is colossal, leaving both subject to knockbackable things, as they are currently the only two single-base colossal-potential characters.
I would also assume that Stranger (at the beginning of the turn after H-M smacks him) could once again choose to become colossal, but I might be reading that wrong entirely.

Correct on everything, except the last part. Stranger could still, technically, choose to be colossal, but Earthbound would keep him until Hulkmariner was gone.

Can you cite some rule that supports that?

That water is pretty cold.

Just Earthbound itself:

Because Earthbound is active as long as Hulkmariner is on the map, and it can't be ignored, then Stranger using his power to become a colossal (or giant, for that matter) wouldn't do anything. But, since nothing in Earthbound would actually prevent Stranger from using his power (since it wasn't actually countered), he could use it and if HM is KO'd, or reverts to Namor, Earthbound would go away and the power would activate.

Nekron's special power pulse wave causes knock back that can't be ignored.

I'm sure there are at least one or two other instances of knock back that can't be ignored.