How long is that thing?
You don't always have a tape measure when you need one. But you know what you do have? Hands, arms and legs!
If you know the length of different parts of your body you can use those measurements as a quick references for measuring anything! For example:
The space between my pinky and thumb spread apart is 7 inches; my arms spread apart is 60 inches; the tip of my nose is 60 inches from the floor; the top of my hip bone is 40 inches from the floor.
These basic body measurements are helpful in lots of ways. If I'm hanging artwork I can quickly tell if something is not at the centerline if the center of the artwork is not parallel to my nose. (Centerline at a gallery is often around 60 inches.) While moving artwork around, if I quickly want to see 2 paintings next to each other roughly 5 feet apart, I can just spread my arms to get that measurement.
And once you are familiar with your body measurements that will help strengthen your overall visual spatial relations.