This is based off of my limited knowledge of housing as it currently works, if someone else is able to provide more accurate information or point out mistakes in my description you'll be better off for it.
Size of house: Kind of self explanatory, it determines the footprint and by extension the space avialable inside your building. This means you can fit more furniture and workbenches inside a bigger structure. I believe a small house with no tiers can hold 4 workbenches, but not sure on that last detail.
When placing a house the game considers the footprint of the building when deciding if your house will "fit" in the space you are trying to place it in. This footprint includes the space required to add all the tiers to your building, so even if your small house deed fits in the area it might tell you there is not enough space for extensions when you are trying to place it. (Something to consider when chosing a house location)
Another thing to consider when placing is that certain POIs have invisible no build zones, and strongholds create an invisible "No TC structure" area of 100m radius if memory serves me right. (The last part about strongholds is not important right now as there are no territory control structures ingame yet.)
Upkeep: I have no idea what the exact upkeep per day is currently on all houses, but I believe that the upkeep for the smallest house with no tier upgrades is approximately 70cuprum coins per hour. Honestly upkeep on housing is currently a joke and you could cover the upkeep for any of the housing structures very easily.
Tiers: Tiers are also very simple to understand once they've been explained. You first need to buy the first tier and once you've finished construction of it you can purchase and add the next tier to the already completed tier 1 house you've placed. So you would buy tier 1 and build it, then you can buy tier 2 attach it to your tier 1 house and build it before buying the tier 3 and attaching it.
Building Zones: Building zones are invisible, the game will tell you in the top right hand corner of the screen if you are trying to place in an area that you cannot build in. For the most part you can build anywhere on the map that has the space to fit the footprint of your house's foundation. No build zones for the most part are in pretty obvious areas, you can't build inside or too close to cities, dungeons, and other important locations.
Hope this helps!