There's really no rule of thumb for building houses...The kitchen should have enough room for all the necessities like the fridge, stove, sink, dishwasher and trash compactor... Bedrooms can be any size but I build them 5X5 or 6X6... Plenty of room for a bed and some other furnishings... The dining room should be big enough for a table with 8 chairs to fit into without the backs of the chairs touching the walls... Living rooms are pretty easy to build and can be any size... The placement, sizes and shapes of the rooms all really depend on the size and shape of the house that you want to build... One thing that you have to think about is the placement of the stairway if you're going to have a house with two or more floors... I try to frame the house and put in the floors and stairs first... Then I go floor by floor installing the inner walls for the rooms. If I find that a room isn't big enough then I just add onto it and just go floor by floor and adjust the exterior walls.
If you're not sure what type of house you want to build, you can find some good examples here:
Post the pictures of sim houses in your game
http://www.insimenator.org/showthread.php?t=24809I started out by modifying houses that were already in the neighborhood by adding floors, moving the exterior walls, etc., before I started building my own...