Rates and scale factors
Lengths scale by k, areas by k squared and volumes by k cubed; a unit rate is the amount for one unit.
A worked example
On a 1 : 200 drawing a 15 cm beam is 15 x 200 = 3 000 cm, which is 30 m in real life.
The mistake people actually make
Doubling every edge multiplies volume by 8, not by 2. After using a map scale, convert the answer into sensible units before writing it down.