For building I work on the basis of 1 tile = 1 square metre, but I know that is not really correct according to the size of objects in the game. Even a square yard would be too large, I think. A more accurate scale would be something like 1 tile length = 80cm (about 2' 7.5"), which is the width of a normal door.
By the way, if you look carefully, you will see that even though beds have a three-tile footprint, they are not actually three tiles long, but nearer two-and-a-half tiles. This would make them about 2m (6' 6") long in RL, which is a bit large, but still fairly realistic. The sims themselves are a little shorter than that - about 6'3" or 6'4" in RL, which is fairly tall by any standard, but not remarkably so. Even so, there are many objects in the game that are horribly oversized. The side tables and dining chairs are probably the worst of the lot. No-one I know has 80cm x 80cm side tables, or 60cm x 60cm dining chairs.
Getting back to building, though, I still find it easiest to allow 1 tile per square metre, partly because I am not about to do my head in with complicated conversions when I am following a building plan, but also because sims need more space to move around in than people do in RL. Similarly, when working with measurements of part of a metre, I find it better to err slightly on the side of generosity.