As far as sizing is concerned, I look at things that are a set size in Sims. Doors, beds, bath tub, toilets, showers, fridge, sinks, stoves etc. Start with a room that has a number of 'set' items like the kitchen and work on that first. I find it helps to put those items I as you build even if it's just a cheap place marker item until you furnish and decorate properly. Of course, some spaces are going to be wider or narrower than the real thing, but once you get the sized things in, the rest usually falls into place. Also, I find it helps to actually plot out the house on paper (graph paper is excellent for that) so that you have a rough idea of size and where to place it on the block.
Hope this gives you a bit of an idea how to start.