Our sweaters are the warmest thing we make. They're cut a little roomier on purpose, so on properly cold days they go over a tee or Long Johns, and that layering beats any single garment.
Each garment's product page shows its Warmth Factor, so you can compare styles at a glance: the higher the number, the warmer it is.
Stick with your hound's usual size in a sweater. The extra room is already built in, so there's no need to size up to layer. Our warmth guide to tees, Tweaters and sweaters has more on choosing the right weight.