I usually use body length only because between long fin and short fin, measurements can be biased. Measuring from outside (like above) may not give you precise measurements. I usually put a half gallon tank inside a gallon tank - betta will be very close to the glass. OR since they won't keep still, I often put them in a plastic bag, reduce water to minimum/flat - then measure.
Bettas keep growing until they're about 12 months. Giants spawns grows rapidly but the half and non giants will suddenly stop growing at a certain size. Instead of adding length, they add width and thickness (a sign that they've stopped growing). The giants will keep growing and IMO should reach 6cm by 4 months. Otherwise they will only be half giant size by 12 months (under 3").
I learned the hard way that where I'm at half giants are considered as giants - big disappointment. And since they are constantly being crossed to improve finnage, most are only half giants, if anything.