Welcome to YT. I'm sorry you lost your little fur-boy in such a tragic way. I lost my 16 year old Chihuahua in Sept. and am sill hurting. We had her since she was three months old.
As for how big your new girl will be, well given the size of the mom and dad, my guess would be this. She could take after the mom and be 10 pounds, take after the dad and be 3 1/2 pounds or somewhere in between. You may know more about her size when she's about 8 weeks old. Make sure she stays with her mom till she's at least 12 weeks old.
Here is a chart that may help you later on.
http://www.yorkiepassion.com/growthchart.htm