Fairies don't do imperial, ladies and gents!
I have no idea what kind of image Red has in his head at this point about humans. He clearly assumes humans start out the same size as him, but just keep growing after that. then stop growing once they reach their gigantic sizes.
Now I'm guessing he tought of a normal sized person (for him) with a human sized foot. No wonder he was a bit upset. It's gonna be a long way until both Red and Ray get on the same page.
Ray: What is going on? (Pumdum) I don't have the slightest clue, I just slapped a cow! (Padabadam)