Tallest person in the world. This is a recurring theme. "When I grow up, I will be taller than this house!"
"I will be taller than a tree!"
"I will be so big, my head will be up in the clouds! The clouds!" (Sometimes he repeats himself to make sure we recognize the utter astonishing heights to which he will climb.)
"I will have to have special clothes, because I will be taller than everybody!"
Lots of kids. "When I grow up, I will have lots of kids. I will have eleven kids! I will have ... nine girls. And just two boys. Two." (Holding up index finger and thumb to signify the number two.)
Huge vehicles. He's suggested bananamobiles (™Richard Scarry) as a possible mode of transportation.
Another possibility: articulated trucks. Y'know, like those huge buses you see around town.
Watches - out. "When I grow up, I won't wear a watch. The fancy people don't wear watches at all!"
Nipples - in!
"Mommy, why do I have two nipples?"
"You just do, honey."
"When I grow up, I will have five nipples. I will have more nipples than everybody!"