Copied from the AKC website dachshund sizes are classified as:
"Size, Proportion, Substance
Bred and shown in two sizes, standard and miniature; miniatures are not a separate classification but compete in a class division for "11 pounds and under at 12 months of age and older." Weight of the standard size is usually between 16 and 32 pounds."

Naturally the best indications of how large the puppy will be when full grown is the size of its parents. The Dam of this litter of pups can truly be considered a miniature as she weighs 10.5 pounds. The Sire of this litter is a small standard as he weighs 16.7 pounds. I would speculate that the pups from this litter will be in that range but that is strictly speculation. Nobody can guarantee the size a puppy will become. Keep in mind you can breed two 9lb dachshunds and still end up with a puppy that is 15-20 lbs as an adult.

If Someone is guaranteeing to be selling you a miniature Dachshund ask them if they are willing to put in writing that if the puppy you purchase exceeds 11 pounds at 1 year of age that they will let you return the dog for a full refund of your purchase price and any vet bills you have incurred during that year.

I have personally purchased 4 AKC Dachshunds. I still Have three of the four. They were all sold to us a Miniatures. The smallest would be the Dam of this litter at 10.5 pounds. The second smallest would be a female we no longer have but one with which I have regular contact with the current owner; She is about 13 pounds. The third is a Female we currently have is about 14.5 pounds. The fourth is the sire of this litter that weighs 16.7 pounnds

Below are pictures of the Sir and Dam.