Is this too big for a dachshund/terrier mix?

Not really, that would make it very comfortable. Nice cushion would make it like a 4star hotel :)

You can order online. They have small, medium and large.

Since an Item # was not provided, we are unable to determine which model you are viewing. The igloo units start at a medium size for pets up to 45 pounds and the barn shape units start at an extra small for pets up to 15 pounds and to up from this.