Roman is an 80 lb Greyhound that stands 29" at the shoulder - large even for a Greyhound. Knuckles is still a puppy and is just a tad taller and 40 lbs heavier.
Though Knuckles being in the foreground sort of exagerates his features compared to the Grey - his head IS larger in width than Roman's body. Knuckles could probably snap Roman in half if it weren't for the fact he is deathly afraid of him, and could never catch him even if he weren't so afraid.
Update, 3 months later, Knuckles is now nearly fully grown at 140 lbs. He eats a LOT of food. And makes a LOT of mess. :( We love him, though.