One of the things I did, if trying to scale walls, doors and ceilings is place the arms up straight over the head. A 6' person will have a slight crook in his arm when touching the top frame of a standard door. So this can give you an idea, (only an idea) of how high a door should be. Use the same concept for cars, bikes, and other props.
A standard desk should come up to the bottom of your buttocks, so therefor, use the bottom of the buttocs on your figure to determine a hieght of a desk.
This system is not 100% accurate, but id does work.