I think that introduces a world of problems with different people interpreting sizes differently, as well as having to track all that for each person each time they rank up.
When we were coming up with the new system, I believe it was intended (in my mind at least) that a building is equal to one building. Some larger buildings will take more work true, but it's theoretically compensated by the fact that those are more rare compared to say hovels... which are super easy to make, and can be thrown up quite easily. It was expected that there would be a mix of large and small buildings however, and not just 25 hovels grouped together. That would be reflected in the rank application thread and was assumed the community wouldn't vote someone through with just a bunch of tiny buildings.
Also, I don't remember anything official on Aldemeria counting buildings as anything more than one building. This was different for vassals being promoted within their realm by a liege, and I even benefited from that a little with Anelthas Castle for my Count promotion.
I'm not against the idea, but there is a large issue with tracking building size on top of number of buildings, which will be challenging in the higher ranks, not to mention having to keep track of and exclude vassal builds that will be mixed in.
I'm open to ideas though.