Estimating length is an important life skill. Not only does it help when you want to have a good idea of how far away something is, or how long, wide or tall some object could be. It is also important in common practical circumstances like moving furniture into a room, how much wrapping paper you might need to wrap a present, or how far apart to plant your seedlings in the vegetable patch.
There are two parts to good estimations involving length:
To get good at these, try and find some baseline lengths that are familiar to you. For example, ask yourself:
Then, when you need to estimate you could think about the estimation in terms of distances you are familiar with.
Choose the best estimate for the length of a classroom.
Estimate the height of a desk.