Pretty small - but not that much smaller than any other random 11 year old trying to kill someone

I'm sure that Harry's suspicions won't be primarily focused on other 11-year olds either. Snape/Quirrel/Voldemort's ghost would be his prime suspects. If and after he's eliminated them, he'll probably move to other professors, and then to upper-classmen students.

If he's in the end reduced to investigating 11-year old suspects, he'd probably still be first considering people like Zabini, or Crabbe, or Padma first. And now that i think of it, Susan Bones too, since who knows whether her double-witch powers are in reality a hint of some dark power possessing her or whatever.