My Dogs boxes are 3 and a half foot long, 2 foot wide and 2 and a half foot deep, but that's housed inside a bigger brick shed, with 6 foot runs coming from that. When winter comes I just chuck in a load of bedding (Rugs and blankets), and they're fine. You'll be surprised how warm they stay, Their bedding is always toasty warm when they get up out their beds.
If they've been worked hard and need to heal or it really is freezing cold, and they're in kennels, then stick a coat on them if you're really worried, they'll be sound .