Layout Options
Graph paper is a popular option for designing your space plan for your new home. There are also free digital programs available, but be prepared for a learning curve if you are not accustomed to using Computer Aided Design Software. As you layout the furniture on your space plan, try to allow 36" walkways and paths between furniture, as this is the desired width for using walkers and wheel chairs. This can be hard to accomplish when trying to get your choice furniture into a room, but at least by starting with 36" spacing as your goal, you can manipulate and move around the furniture on your layout to get as close as you can to the 36".
Additional Option
Painter's tape can be used in addition to, or in place of, a printed space plan. At your new location, placing painter's tape at the corners where you plan to place each piece of furniture can give you a closer to real-life visual of how the furniture will fit into the new home. Additionally, because you are downsizing, it is likely that a particular room is smaller in the new location than what you currently have. In this case, you can use painter's tape to outline the area of the new room within the current room. This allows you to see the actual pieces you currently have within the smaller space.
Measuring
When measuring your furniture, I suggest you get width, depth, and height. In can be tempting to skip the height, but it seems like there is always an unexpected situation where you need the height of some pieces, and it only takes few seconds to go ahead and get it while you are measuring. As you measure your new home, do not forget to take note of the direction the doors swing. Depending on the heating and cooling being used in your new home, being aware of any registers or separate units may be important as well. You may need to find out what connections are required for TV and internet, and measure where they are located. Don't forget switches and thermostats, which can sometimes be at unexpected locations.