I'm not too experienced with designer (cross?) breeding... but, I always thought you weren't supposed to breed two designer breeds together?
Because, I always thought that when you did so- the genetics would be different than breeding a poodle with a yorkie. When it comes to poodle x yorkie, guessing the weight, size, hair, etc of the puppies is easier... as both dogs in the breeding are purebred and their genetics are more definite. But breeding yorkie poo x yorkie poo is breeding two mutts together. Meaning that it would be very hard to guess the genetics of the resulting puppies. Meaning that- their size could be bigger (or smaller) than the parents. Which, might explain why your puppy might be bigger, if that is the case.
As I said, though, I'm not experienced with cross breeding; that's just something I read off of a breeders website who bred cockapoos. She admitted to never breeding two cockapoos together and elaborated on why. Genetics.
Well, you double at 3 months and triple at 2 months. So, being about 3 lbs now... your dog will probably be about 9 or 10 lbs. Of course, some dogs are a lb more or less. But, you get a close estimate from doubling and tripling.