When the 2 Buzz, the potato, the dinosaur and the pig are in Al's house, the "true" Buzz is talking to Woody. You can see that Buzz is tall almost as Woody. After Buzz says, "You're a toy", you can see that he is very shorter than the Cowboy!!
This is not a slip-up. Buzz is always shorter than Woody. When he says, "You are a toy!", he is on his tip toes, making him look as tall as Woody. When Buzz goes back down to his heels, he is no shorter.