Built another small house for two sims yesterday, 7x10 i think (it isn't a contest house). Due to windows 7 support ending, i've moved across to Kubuntu and that means unless EA starts supporting the Linux OS a lot more, my spell is effectively at an end and as a result i can't check to see what the exact dimensions are, but i know that i used all the space i could and it is on the gallery as 2 sim very basic house.
I might have another solution in hand with an old laptop, it will take some time to get it ready to play TS4 though. i can't even be sure it will even work at this stage. I've never had need for it and it's just sat under some boxes in storage all this time. IIRC it has a a dreadfully slow 5400rpm mechanical drive, so that needs urgent replacing!
anyway, you'll get the hang of it, it's just about building confidence with it, expanding the boundaries and pushing the limits of what you expect.