Forgot the tape measure!
Bernie, if you know how tall you are, lie on the floor with your feet touching the wall and measure out your length, mark it, get up and lie down again with your feet on the mark, and so on until you know how many Bernies long and wide your room is. Then you can calculate room square feet/metres. Make sure you get someone to video you doing this and - this is the most important bit - be sure to post it on CBF. Hope this helps, Colwyn.