If my opponent has a full hand of 7 cards and I cast wistful thinking, what happens?
Do they discard the two cards they draw and then discard four more for a total of six cards discarded?
No, if you cast that spell on your opponent, he draws 2 for a total of 9 hand cards, then discards 4 cards, down to 5.
402.2. Each player has a maximum hand size, which is normally seven cards. A player may have any number of cards in his or her hand, but as part of his or her cleanup step, the player must discard excess cards down to the maximum hand size.
The cleanup step is the last step in each player's turn.