Size is measured by the height of the middle-zone letters like a, e and o. Larger than about 3mm is large. Around 2 to 3mm is medium. Smaller than 2mm is small.
Large writing often belongs to people who enjoy being noticed and speaking up. Small writing often belongs to careful thinkers who prefer depth over volume.
Watch consistency. Writing that shrinks toward the end of a line can point to shyness that grows with fatigue.