My Malshi Puppy Is 3.5 Lbs At 8 Wks. Mom Is Shih Tzu And Is 8 Lbs; Dad Is A Maltese And Is 7 Lbs. How Big Should My Puppy Be When Full Grown?

It might save time to just say your pup is a Maltese/Shih-tzu, Malshi is a little confusing and then you will have to explain usually anyways. :-) It's difficult to predict growth in a younger pup sometimes (better to try in 2 or 3 months in this case probably) and more so in a mixed breed. The shih-tzu is definitely on the smaller side for the breed and may come from a line of mostly 12 to 16 pound dogs. The maltese may even have larger dogs in their line and you never know how the genes are going to mix. Maybe around 9 to 14 pounds or so. The important thing is your pup is a good weight for their individual self and body type and you love them. :-)
Your pup has both sets of genes from not only both lines but both breeds. Mixed breeds can take any traits or combination of traits from the breeds involved with even litter mates ending up completely differently from each other. They don't have set genetics that lead to predictability like a pure bred you are mixing the genetics from more than one breed and widening the possibilities. You would study up on both breeds to learn some of the possibilities and you can try looking around on petfinder for some same or similar mixes to get some ideas. Congrats on your new puppy.
www.petfinder.com  
www.americanmaltese.org
www.americanshihtzuclub.org  
home.comcast.net  
www.learntobreed.com
www.terrificpets.com  
www.wildheirlabradorrescue.org   
www.dogfoodanalysis.com  
www.bornfreeusa.org