Hi Francesca
Thanks for your question.
It's great that you eat healthily and I would always concentrate more on what you are eating rather than being too worried about portion sizes. For instance, I would ensure that you are eating lots of non-starchy vegetables, some healthy fat with every meal such as avocado, olive oil, nuts, seeds, oily fish (e.g. salmon, mackerel, sardines, herring) and some lean protein such as chicken, eggs, fish or plant proteins such as beans, lentils, nuts in order to keep blood sugar balanced and energy levels stable. However, if you would like to have a better idea of your portion sizes there is a guide below plus to give you a rough idea of what your plate should look like I would recommend half of it contains vegetables or salad, a quarter of it lean protein and the other quarter complex carbohydrates such as quinoa or brown rice and some healthy fat. Here's the guide I mentioned. Hope it helps.
Fruit: 1 medium piece e.g. apple or ½ cup (125ml/4 fl oz) fresh fruit or ¼ cup (63ml/2 fl oz/small handful) of dried fruit
Grains: 1 slice wholemeal bread or 2 oat cakes
Starchy vegetables: ½ sweet potato or 2 small new potatoes or 1 cup (250ml/8 fl oz) butternut squash/pumpkin
Non-starchy vegetables: 1 cup of raw vegetables or a large bowl of green salad leaves or ½ cup (125ml) of cooked vegetables
Meat/fish/chicken: 115g/4oz (size of a pack of cards)
Wholegrains: 1/3 cup (85ml/3 fl oz) cooked (small fist size)
Beans/lentils: 1/3 cup (85ml/3 fl oz) cooked
Nuts/seeds: small handful