Agreed, there's nothing intrinsically wrong with jokingly calling kids "fun size," and it's not sexual.

However, I was going to say I didn't know the context in this case, but then it popped into my head; I believe this is that scene with the child version of Poison Ivy...i.e. someone they love to pair with Harley. At the very least, she's calling a child version of someone with which there's presumed sexual tension "fun sized." Honestly, it would be much more excusable if it were literally almost any other pairing of two characters. As is...at the very least it's a little suspect.

Harley delightedly calling a child version of her sometimes girlfriend "fun size" is...a bit off.