In the UK we normally refer to a 9-inch/12-inch pizza, for example (and it's understood that the size refers to the diameter).

A pizza of 20 centimeters in diameter.
OR
A pizza with a diameter of 20 centimeters.

Sorry, but #2 is what a non-native speaker might say.

"Find the circumference of a wheel with a diameter of 50 cm"

Reference: http://www.bbc.co.uk/schools/gcsebitesize/mat...

AE also has 8-inch or 11-inch pizzas, e.g.

#4: Context is everything. This is about pizzas, not maths.

What Spike said.

It's a 20 cm pizza (or if you like: a 9-inch pizza)

or (depending on context, and more literally)

The pizza is 20 cm in diameter.

Tausend Dank, ich glaub jetzt weiß ich Bescheid:)

Übrigens klingt "Die Pizza hat einen Durchmesser von 20 cm" auch im Deutschen nach dem Beginn einer Mathematikaufgabe. Der Pizzabäcker ums Eck würde sagen: eine 20 cm große Pizza.

20 cm is less than 8". What is this, a single-serving pizza? I'm not very familiar with pizzas that size. Other than pizza-by-the-slice, pizza is usually ordered here for multiple people. My local place sells 10" (25 cm), 14", and 16"; The next one over has 9.5", 12", 14", and 16".

Not sure what the problem is. Over here we have one-person 7.5"- or 8"-pizzas.