One way to gauge your appropriate portion size without any measuring tools is by simply using your hands. As your hands usually correspond to your body size, bigger people who require more food typically have bigger hands ( Source).
A rough guide for each meal is:
High-protein foods: A palm-sized serving for women and two palm-sized portions for men — such as meat, fish, poultry and beans
Vegetables and salads: A fist-sized portion for women and two fist-sized portions for men
High-carb foods: One cupped-hand portion for women and two for men — such as whole grains and starchy vegetables
High-fat foods: One thumb-sized portion for women and two for men — such as butter, oils and nuts