Source: http://na.finalfantasyxiv.com/lodestone ... de31482c6b
Summary:
Size is slightly bigger than an Inn room. Same size regardless of house size.
Costs 300,000 gil for each room.
One per person presumably, but it doesn't say that, it says the room limit is 512.
Furniture item limit is 50.
Floor/walls/lighting customizable.
Able to set greetings, guest access, and even rename the room.
Your room can be set to allow or disallow visitors.
You dismantle the room by removing all the furniture, but you won't get your gil back.
So basically, it lets you have your own room in the FC house that only you can edit, and set up however you'd like it. This is not the same as having a future house to yourself, but it allows you to have the benefit of playing with as much furniture as you want, without having to shell out the millions of gil for an actual house.