I wouldent worry about the size

Lady is only 2lbs and Jess (my second yorkie) is over 7lbs and they get along fine

Infact,Lady seems to be the alpha in this yorkie relationship! lol.Lady was a pound and a half when I got Jess and Jess was about 2lbs at 8 weeks old,they basicly ignored each other for the first 6 months! lol.
Dont worry about your new babies size

How ever big or small he may be,they will end up loving each other