It depends on what kind of chili they are, Milo. The big New Mexico types can get up to 25 cm long before they start to turn red, while 2 1/2 cm is pretty big for a 'Serrano'. Usually, If you want green chilis, you let them get as big as they can, because they usually taste better that way--really young ones sometimes have too much of a "grassy" taste. You can let them turn red or yellow, too, but that's a different flavor, though good also. I would check the seed source for size at maturity.