I need someone to measure a mattress for me, but he says he can't because he has nothing to measure it with. I'm currently about 800 miles away from him, so I can't just take the bull by the horns and drive over there to help.
Is there a trick I can suggest him to use, which does not involve any measuring tools? All I need to know is whether it's roughly 140, 160 or 180 cm wide. I don't need the measurement to be very precise.
You could compare it with the size of something that has a standard size. For example a CD/DVD is 120mm, so you could count how many times you can flip one over along an edge and get a rough measurement.
If you really only need to distinguish between 140/160/180cm, just have your friend lay across the bed (with feet/head lined up to one side). Assuming you know (or he tells you) how tall he is, he can say something like "it's up to my shoulder/eyes/chest," and that's really all you need.
Get your friend to measure it in sheets of a toilet roll and tell you how many. For the accuracy you need, even the brand won't be important, but you can be doubly sure by buying the same brand where you are then measuring that length yourself using a tape measure.