I am picking up my first Yorkie puppy on WED

We will be getting the female in about 3 more weeks. He is 1 lb 6 oz at 8 weeks-anyone know if this is considered small or what an estimated adult size will be?
I wondered what you all give your yorkies for heartworm preventitive. The breeder told me that he has had yorkies for 32 years and never gave them any -that it has been known to be unsafe for smaller dogs and especially yorkies. I used to work at a Vet. hospital, so I know how devestating heartworms can be. I am torn on what is the right thing to do. Any advice would be greatly appreciated.
Heather