So's I'm a big Grant Morrison fan and I love wizkids new take on ol' Buddy but his Morphogenetic Field trait brings about an interesting possibility.

If the new Animal Man were fighting Starro would he be able to copy his Colossal ability?

I asked a similar question in the Rules thread regarding the Multiattack ability (ruled as legal), so using the Colossal Size ability should also be legal.

Honestly, you're better off trying to hit Starro for 12 using B/C/F in two attacks with Multiattack. If you're facing the Rookie version and have Time Trapper or Skrullowjacket and a few other Outwitters, you could potentially wear him down in a turn or two (counter Quintessence; Outwit Invulnerability; attack twice; Outwit Toughness SP; attack with someone else).

He could copy Colossal Size, yes.