According to the YTCA site, it just says that weight should not exceed 7 pounds
Yorkshire Terrier Club of America
And I double checked the AKC site and it looks like it says all the same things
American Kennel Club - Yorkshire Terrier
I have frequently heard that standard is 4-7 lbs as well but don't know where people get this....
Now if you were to show a 2 or 3 lb dog, it probably would not do well because the real purpose in showing is to evaluate breeding stock and dogs that small are not good breeding prospect.