Some more information about model scales:
Models are expressed in scales relevant to the real car.
For example a 1:18 scale model measures 18x smaller in all measurements to the real car.
The most collectible sizes are 1:18, 1:24, 1:43 and 1:87 (HO).
The examples in the above pictures are as follows
The BMW Z4 is scale 1:18
The Aston Martin DBR9 is scale 1:24
The Mercedes C Class is scale 1:43
The Ferrari Super America is scale 1:87 (HO)