Hello @mikeharpe,
This is part of the reason for this article. Now there is certainly a common size for many resistors. Take for example 1/4 watt resistors. Of the 51,000 1/4 watt through hole resistors currently in our system you will find that 32,000+ of the options fall between a diameter of 2.29 and 2.5mm and a length of 6 to 7mm. This being said you will also find a about 7,000 that have a diameter of less than 2mm and a length of less than 4 mm.
So no, there is not a standard that says if you order a 1/4w through hole resistor it will be 2.29x6.35mm. This is why it would be important to look a the data sheet for the resistor’s physical size prior to ordering if you are looking to ensure fitment.
This is also important to look at if you are replacing an existing resistor as you will not be able to say that the resistor must be 1/4w because of the size because while 2.29x6.35mm may be a very common 1/4 size you will find options in a 1/2w that will replace it.
If you have any further questions please let me know.
-Robert