My dogs are a good size. They average between 60 to 85 pounds. And I gave them a heaping spoonful after they had soft stools from liver. It firmed them right up.
But I don't think there is anything bad in pumpkin besides extra calories. Your dog should be able to eat the whole can without any ill effects.

But I would start with 1 or 2 tablespoons and see if that solves the problem.