Prof. Aragorn, a character with Charge ignores knock back. Characters with Great Size can't be knocked back. There is a difference.

Hmm. Forgot this one:
Aerial baffler: Any character with the {wing} or {wing-transporter} speed mode that is 3 or fewer squares from this object gains Earthbound. Any character is carried takes no damage and is placed in the nearest square it can occupy.


Earthbound/Neutralized:
This character can't use combat abilities and possesses instead of its combat symbols. This power can't be countered or ignored.

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 we go back to this? Earthbound does its thing to Stranger or Groot, say. But like EB, Great Size says it cannot be countered. So in the example above, wherein Stranger can choose at the start of the turn after H-M bashes him, can he "do what colossals or giants do" by choosing it, even though he would still have the from H-M's special power?
I would tend to think the latest power (Stranger vs. H-M) would be in effect, but also H-M's power is doing something to Groot that I don't think it was intended for (knocking flyers out of the air).

You have the answer in your own post it was just hiding...

Earthbound/Neutralized:
This character can't use combat abilities and possesses instead of its combat symbols. This power can't be countered or ignored.

Stranger would have Colossal Stamina and Great Size, but he could not use them.

I also think that the 'instead' from Earthbound would overwrite any new symbols he gains as well.