According to the British Dietetic Association, a portion is: a large slice of melon or pineapple; a whole apple, orange or banana; two plums or satsumas; a cupful of berries or grapes; a tablespoon of dried fruit; three tablespoons of tinned fruit; a 150ml glass of fruit juice; a dessert bowl of salad; two tablespoons of any other vegetables.
If you can count to five & consume five fruit or veg every day, you must have lots of money and a balanced diet: so then the potion size doesn't matter.