Heli size question

How do you determine a heli size... I've seen 400, 450, 500 etc..

Is it by the heli's length or rotor's diameter? If its the diameter,
then am I right to say the HBFP V2 is a size 500 since its rotor diameter is 510mm.

hbfp v2 i believe is a 300 size, blade 400 or g2 raptor at xheli is a 400 size, belt cp, blueray, is a 450 size, exi 500 is obviously a 500 size check them out at nuttcazes youtube channel, he has the heli comparison there, this will give you and idea of sizes...