At what puppy age do you usually know how big it's going to get?

Our family is looking for a Boxer, Teacup Yorkshire Terrier, or maybe a Mastiff. We have 2 toddlers and a responsible 10 year old. We also have a hairless cat, Canadian Sphynx, which is one of the least worries for she is the nicest cat in the world. She never scratched, bit, scared, not even attacked us. She is just an adorable sweet heart. We know it is dangerous for a Teacup, but we aren't going to let it wander around the whole house. And it is most likely going to be a portable-handheld pet. We have experience and know what we're doing, but it's most likely going to be a Boxer. A Mastiff is one of our last options. I hope you understand because we had one of these problems before. One of our family members saw a Toy Poodle Ad in the newspaper. Payed a fairly very large amount of money for a Toy, and turned out to be a Standard Poodle. We don't want to make the same mistake again. Please give me your opinions, recommendations, suggestions. Thank you; 10 points to best answer. Jackie