I am looking for a code that will define the largest permitable hole diameter for under eave ventilation (Soffit Vents). I’ve heard that too large a opening will allow insects to enter the attic If possible I am specifically looking for and ASTM code number, Thanks
There is no limit to size mandated by any code I know of. They do put a minimum mesh opening for screening the vent hole. 1/4 x 1/4" it is. It isnt there for insects it is there to keep varmints out. There are ways to vent more efficiently that would basically say a bunch of smaller holes spread out accross an area are more efficient than a few big vents here and there.
Industry standards is one thing, code enforcement is another. Industry standard vary by area. Venting calculations should be done by an engineer. I believe it is 1 square inch of ventalation per 150 cubic feet of attic space / crawl space or garage for cumbustion air, where im from. Call your local building department.
Why would you want to put round holes in the soffits rather than using vents? There is nothing that i know of either in this matter.
Drilling 3 -4 " holes is a common practice out here in the west. The term soffit is used loosely as well ( outside the profession that is) and doesnt always mean it is closed in. You can thank do it yourself tv for that, and a workforce that barely speaks english anymore.
It is easier to drill multiple holes in a soffit finished in Stucco.
just remove the old soffit board,on new houses we never put wood,if vinyl soffits are going in-never had a problem w/varmits yet 
I should be more specific, I am redesigning a pultruded soffit/gutter combination, the easiest way to have soffit venting is to cut the holes at the factory, as samskii mentioned it is best to have lots of small holes scattered over a large area. I was told about their was an insect code, and am having no luck finding such a thing, it might just not exist. I have heard rummers of a code for fires in California relating to hole size, have any of you heard of such a thing?
do you mean yankee gutter?the gutter trough is actually part of the roof,you should be sure what you are cutting is not the roof sheathing as well,if so ,you need to drill the holes at the juncture between the rafters,so it vents the attic—this is why it`s better for you to post pictures