Ford Fiesta car requires a set of wheel trims which are 14 or 15 . True or false?

For a small vehicle, which one is more likely: 14 or 15 inches?

Look on the sidewall of the car tyre it will read something like '195/55 R16 87V' The number that denotes the wheel size is the third from the left, in this case 16. That will be your wheel size and the size of the wheel trims that will fit the Fiesta.

"wheel trims"? What is that? If you have plain steel rims, then maybe the vehicle has or had trim rings… The answer from anon tells you about the tire size

A late model Ford Fiesta comes stock with either 15 or 16 inch diameter wheels and tires.

Depends on the age of the car. The Fiesta has been present in Ford's model range in western Europe since 1976.

The first Fiestas had 13" wheels, with 14" appearing on some versions of the mk3 around 1989, and becoming the base wheel probably with the introduction of the 2002-2009 model.
The latest generation has 15" wheels or larger, depending on trim level and options fitted.