The short answer to this is yes. I will have to compile a list of different sources of standard lengths to get a better picture of what that might look like. That should not be that hard to do put that in a table to pull from. It might even simplify things a bit, especially in the BOM. I would however, still allow the edit of those values in the edit part button in case there is a crazy length not listed. It would also keep user from entering a 800mm hex bolt. I say this cause we get some really weird sizes sometimes from factories around the world for some reason. That list should knock out 99% of what people would expect. This same thought also could be the same for shank length as well. I will look into that too.
Thanks for the suggestion 
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I have resisted mentioning it, but I still haven’t forgot about your suggestion on a custom CSV file feeding the bolt and screw sizes per your inventory. It’s been in the back of my mind on how to implement that. My thinking is that I could have python do a lookup on a CSV file when the plugin loads. That CSV would feed and populate the sizes you have in inventory. That CSV would could hold your fastener attributes like part numbers and quantities which would feed into the BOM. If nothing is listed then it would fill in with my table of fasteners. This is all in theory at the moment and not sure it would even work, but I think it might should… maybe. NO PROMISES! I find it an interesting concept anyway.
Maybe after I got this new interface project released and the major bugs worked on, I could circle back revisit the idea more in the future.