length, area and volume factors

it's probably helpful if students appreciate what happens to the overall size of a shape if you e.g. double in two or three directions*

a ppt is here

see also big sprouts and Frodo Baggins
and wombats and diprotodons 

*an estimate of the adult height of a male is obtained by doubling their height when they are 2 years old
for a female height estimate, it is double their height when they are 18 months old