Dog sizes acceptable for whelping?

Question
I have a female 5lb yorkie/Chihuahua mix a a 10lb Registered Maltese. Is he too large to breed her? Also, he is 6 months and doesn't seem to want to mount her. Is he possibly too young? She is 8 months and this is her first heat.

Answer
Hi Deb

I KNOW you're not telling me you are trying to breed two puppies together.

8 months is to young for the female to breed, and a males isn't producing viable semen at 6 months.  

Let them both mature before you start breeding, especially a larger male to your smaller female.