[Gayle]

You work out the size of a tree by measuring the distance along the ground to the tree and then from a fixed point away from the tree the angle to the top of the tree. But again, you actually need to be able to visit the tree to get an exact calculation.

If you hold your thumb up in front of the TV, your thumb is bigger than the TV - but does that mean that you've got a really big thumb, your TV is very small or that your TV is a long way away. My point being that you know, by measuring it, the exact length of your room so you can calculate the answer. With space, you don't have one fixed measurement so a planet could be really small and quite close or really big and a long way away.