How big are buildings?

This may seem an inane query, but in our gaming group we're having a disagreement over how much terrain can be modified by purchasing defensive improvements to 'one building'. I was sure I had seen a guideline somewhere that buildings fell into three size categories:
Small Buildings — no more than 6" in any dimension
Large Buildings — exceed 10" in at least one dimension
Buildings — those not fitting into either of the other two categories
But apparently I imagined the whole thing. Even if I didn't imagine it, those definitions wouldn't be very useful as 'small' and 'large' buildings are only mentioned once or twice each, nearly every reference being simply to buildings'.

So, the issue is that we're playing a 'big game' on a 6' by 8' city board. There are a couple of big buildings in the 12" by 20" or 15" by 15" range and one group of three smaller buildings on a common base. There are many perfectly normal-sized buildings, and there are a few tiny buildings that look like garages or outhouses that tend to be close together. One player (who contributed about half the buildings, many of which he made himself) says that a building is a building, no matter how large or small, and that multiple buildings on one base count as one building. Some of the rest of us (who complain a lot, but contribute little) feel that there should be some kind of limitation on how much building gets modified when the defender pays for a reinforced cellar or a bastion building (either of which greatly improves saving throws).

My suggestion is that if a building exceeds 10" in either length or width it counts as multiple buildings — as few as possible, but so no section exceeds 10" in any dimension. Likewise, if two or more small buildings are close together they could count as one building for improvement purposes, as long as the total square inches doesn't exceed 36. And the number of buildings on a base isn't relevant.

My question(s) therefore are:
1) How do you people define a building for these purposes?
2) If you don't have a definition, do you prefer that of the contributor (a building is a building) or the complainer (the Pentagon is five buildings)?

Thank you for your time.

This subject has to be decided prior to the startof the game with th eother players.
There are no offical rules.
Certianly common sense dictates that large buildings should be divided in "sectors".
Some official rules regarding this are detailed in the main rulebook (urban combat) and in the new "stalingrad" supplement.