Lewis - the mantra, especially at this early stage, is IF YOU'RE HUNGRY, THEN EAT! Obviously you mustn't eat carby things. Chicken drumsticks, with lovely crispy skin, would make a perfect snack.
The key thing in the first couple of weeks is to get your head around the whole thing, get through any carb withdrawal symptoms whilst making sure that you're not hungry.
You honestly don't have to worry about portion sizes, as long as you're sticking to the rules and making sure that the vast majority of your carbs are coming from vegetables and salad.
Also, if you keep finding that you're hungry, it's an indication that you may not be eating enough fat.
How are you finding it/feeling so far?