In one shot Topanga has a regular pencil and Angela says, "Why is Shawn so obsessed with you two?" Topanga responds to her holding her big pencil.
Eric claims to be the oldest member of the group and Jack corrects him, stating that he is actually the oldest. Given the characterizations of the characters, it's strongly implied that Jack would know and Eric wouldn't. It was established in an earlier episode that Eric is 8 months older than Jack.