The size have nothing to do with backproblems. Its the dwarfism (short legs) that causes fragile discs. In sweden the smallest dachshunds are more exposed to backproblems.We choose a mini because Standards seem to have more back problems due to their length. Mini's usually have a better temperament than the standards.
This is why I have a standard as well.We have 3 sizes in sweden. We thought that standards are more "dog-like" in their apperence. The way they look, move and they dont seem as fragile as the smallest dachsies.