Over all, if you're fine having a few "N/A"s in you database it sure doesn't hurt anything to add in driver size.

No, it would not hurt if some were n/a; and yes, planars, hybrids and other non dynamics would have to be somewhat marginalized, but not fully. The system i'm working on allows to make an attribute "preferred" which does not filter out items which don't have that attribute, it simply adds bonus points to the ones which do have it. It's sort of like, hey, it would be sweet to have this feature, but if there are products which do not have it, yet preform better in other areas, i still want to check them out, specially if they get good scores. That said, personally i would not add driver size in combination with important attributes such as great mids, great soundstage and perhaps some bonus points ("preferred") to the open design in order to find cans which will make all the jazz vocals sound as good as possible. But if someone wants to look at headphones which have big drivers, i'm now convinced, they should be able to do so.