A rough guide is how much it weighs, with an average being 0.7 - 1.2 kg. However, a small guinea pig that weighs 1.2kg is likely to be too heavy, because of its small frame. The guinea pig should look sleek and glossy, but without rolls of fat or a dewlap. You should be able to feel its ribs when you apply gentle pressure, and be able to easily identify where the rib cage ends and the belly begins.