I follow floor plans and usually I look at the windows and doors and figure out which ones I will use by getting into build mode and seeing how big they are. Then I use grid paper to draw it out.
I don't worry about room dimensions very much. I mostly just look at the shape and try to get it to look as close as possible. But windows and doors are a good guide. I have also used beds and counters etc. to figure out how big I need the rooms to be. I usually I have some idea as to how I want to decorate the build before I put it down on paper. It is getting to the point now to where I know how big most beds and counters are so I don't have to spend as much time going into build mode.
I also look at the exterior of the house and try to copy it. I do that before I worry about how big the inside will be. You can always move things around. I even put in the closets at first. Sometimes I keep some of them and other times I want the space so they get deleted. I love building. Good luck with your floor plans. I hope they show how the house is supposed to look. That helps a lot.