Learn Polish: Podwójny format jest najdroższy. - The double format is the most expensive.

It is built from the root for two (dwa), combined with the prefix po- and the suffix -ójny. You will see this exact same pattern in potrójny (triple) and poczwórny (quadruple).

Why do we use the adjective podwójny here instead of the adverb dwukrotnie?

Because it directly describes a noun (format), so we need a multiplicative adjective that matches its masculine nominative singular form. You would use the adverb dwukrotnie (twice or doubly) to modify a verb, for example if you said the price increased twice.

I remember the word for expensive is drogi. Where does the -ż- in najdroższy come from?

When forming the comparative of drogi (expensive), the g mutates into a ż, making it droższy (more expensive). To get the superlative form used here, you simply add the prefix naj- to that comparative form.

Can podwójny be used for abstract concepts, or just physical things like a format?

It is used for both! Just as it means a double size here, it is heavily used in abstract expressions like podwójne standardy (double standards) or podwójne życie (a double life).