I usually start with cutting the dimensions in half. For example, if a living room on the floor plan is 12x14, I do 6x7 (grid squares). If it's 13x15, I still do 6x7 (since we can't do half-tiles). I then adjust accordingly. Of course, I usually don't follow the floor plan exactly. I just use them to get a basic shape of the house. It just seems easier (for me) to do it this way. But, as someone else said, another easy way is to look at the window placement. It also helps if you know what the sizes of windows and doors are in game.