i was actually thinking about this the other day, everyones definition for a big house is different, here my definition of a big house would be something most people refer to as mansions.
Wall and ceiling material also plays a part, my house is about 186 sq meters, placed on 1204 sq meters of land. I have OK reception even outside of fence, on street.
But my house is very "radio transparent" - wooden carcass, plywood and wool isolation. There is no heavy floor materials between floors.
Also how many rooms house has. Every plasterboard wall only eats about 3dBm of signal while heavy concrete wall with iron armature inside it could eat up to 30dBm.
Router placement also plays a part - if you put it in the corner, near outside wall you won't get good signal at the opposite part of the house.
The best placement for wireless would be of course "chandelier-like" - on ceiling, in the middle of the room. Of course that can't always be done.
There is a software, commercial though - Ekahau Site Survey, it has non-commercial free version called Ekahau Heatmapper which will allow you to generate wifi coverage heatmaps. Only issue is that it requires an compatbile wifi adapter which can be placed in monitor mode.
Just load your house plan/land plan into it and walk arround, marking your path on the plan as you move. Will give you some picture how wireless signal is distributed in your environment.
Plus, if you have neighbours close to you what they do also plays a part. Some idiotic analogue video camera with wireless transmitter can completely block 2.4GHz range and make it unusable, i have seen this on customer sites.
Microwaves, Bluetooth devices, wireless garage door opening remotes - all of those work in 2.4GHz range.
Wireless environment arround us is not constant unless one lives in the forest one owns.