Since it may be awkward sometimes to ask the person standing next to you, "how long is it?", it may be best to develop the skill of being able to estimate length.
This is called eyeballing where distances are measured in how many human eyeballs it takes to span the object being measured. A measurement is often demanded when you see, say, the Loch Ness Monster, so you need to practice with various items like sticks, snakes and trains held at various distances. It is not recommended to eyeball moving trains or snakes at too close at distance, as your estimate can be distorted as well as your skull. Then you just take out your jar of eyeballs strung on string to take the actual measurement to compare.
Once you achieve an accuracy rate of plus or minus 300%, it will be considered good enough for newspapers (in the case of the Loch Ness Monster) and most other businesses.