I've long been under the impression that a quarter-watt resistor has a standard package, and a half-watt resistor has a standard package, etc. But I've recently gotten through-hole resistors which are not in the expected package size. I've done an analysis of a few thousand of the top stocked through-hole resistors at Digikey, and I see no pattern.
3.5 +- .3 mm length seems to be one grouping, which I would normally have thought of as a 1/8W resistor. But Digikey has resistors listed in this size range of anywhere from 1/8W to 1/2W.
6.3 +- .3 mm length is another grouping, which I'd normally call a quarter-watt resistor package. But the wattage is anywhere from 1/8W to 1W.
Similar groupings can be observed around 9mm, 12mm, 15mm, 18mm, 22.2mm, 26mm, and 45.2mm. This makes creating our internal part numbering scheme problematic; we can't just assume a quarter-watt resistor is in a "quarter-watt package", if such a thing there be.
So is there a set of size standards for through-hole resistors? And is there any relationship between those size standards and the wattage of the resistor?