Learn Croatian: Poznaješ li dječaka krupne građe s kojim radim? - Do you know the boy of large build with whom I work?

This is the Genitive of quality (Genitiv svojstva). We use the genitive to describe a specific characteristic or attribute of a noun, translating roughly to 'of a large build' in English. It is a very common way to describe physical traits in Croatian.

Dječaka is in the accusative case. Why doesn't krupne građe match its case?

A Genitive of quality phrase acts like an invariable tag attached to the noun it describes. Even though dječaka changes to the accusative case because it is the direct object of poznaješ, the descriptive phrase krupne građe stays strictly in the genitive.

Doesn't građa usually mean 'material' or 'building'?

Yes, in other contexts it means construction material or structure. However, when talking about people or animals, građa refers to their physical 'build' or physique. So, a person krupne građe is heavily built.

Why do we use the instrumental s kojim here?

The relative pronoun koji agrees in gender and number with the noun it replaces (dječak, masculine singular). However, its case depends on its grammatical role in the new clause. Because the speaker works with the boy, we need the preposition s followed by the instrumental case (kojim).