Taking real life room measurements . . .

When I used to use real life floorplans for building, I would cut the measurements in half, as a general rule. To use your example a room that in real life was 10x12, I would start with 5x6 and adjust accordingly. I also would look at the doors and windows of the real life house and kind of visualize how many in-game squares they would be. For example, if the house had a window that had shutters, I know the game windows with shutters would be at least 2 squares wide. Once I got the basic structure of the house, I would, once again, adjust accordingly until I was happy with the size. Not an exact science but it worked for me.