I use this site. It has been pretty accurate with my puppies.
Find out how big your Yorkshire Terrier puppy will grow - Puppy Weight Estimates
I put in the 2.5 lbs at 10 weeks and it came back saying 5 lbs 11 oz. for you puppy.
Mine said about 8 lbs for one of my puppies and 9 lbs for the other. They are 9 months old and one weighs 7 1/2 lbs and the other is 8 lbs and I think they will still fill out a bit and be pretty close to that weight as adults.
I think yours will stay small. My babies were 3 and 3 1/2 lbs when I brought them home at 10 weeks.