dextrose is PURE sugar, so a pure carbohydrate. 4 calories to a gram of dextrose.
there is no set 'serving size' of dextrose...its whatever you need to get your carb intake goals.
on average, dextrose in a PWO shake is twice the protein amount. assuming 25g of protein, that's 50g of dextrose, which is 200calories in carbs (and 100 cals in the protein, not including any carbs or fat in the protein powder...so prolly more like 120cals from the protein).