Generally yes, I think you're right. Automatically assuming size assumes that your getters and setters are in proper byte order (eg. no bytes skipped when specifying indexes), and might slow down the creation of objects by having to parse the BuiltInValues list and calculate the value. Other than that, I don't see why it wouldn't work.
Maybe if BuiltInSize is blank, it could be determined automatically, but if it's filled in, it would use the defined value?
Perhaps an official response from animeaime would be better, however
