We have a miniature dachshund and shepard (mixed). We were wondering if bred together, will the dachshund die because of the size difference?

Are you asking if Andre the Giant had sex with a midget, would his big cokc rip and kill the female midget? I am not sure if the pain would lead to death of the smaller animal.

not with the actual breeding but the pregnancy and the birthing could..

Why would you want to breed them anyway? Yes, it could kill the female, very easily. I've seen it happen (not my dogs). Your best bet is to alter both dogs to be sure they don't breed, and to avoid them getting uterin or testicular cancer as well, which is unfortunately fairly common.