Learn Hardcore Polish: Który rozmiar wolisz?

Który = which (masculine singular; here matching the noun)
rozmiar = size (masculine inanimate; here functioning as the direct object)
wolisz = you prefer (2nd person singular present of woleć “to prefer”) Polish doesn’t need a helper like “do” for questions, so there’s no equivalent of “do you.”

Why is it który and not jaki?

który = “which (one)” when choosing from a specific, known set (e.g., S/M/L on a label or options in front of you).
jaki = “what (kind of)”/“what” when the set isn’t specified or you’re asking more generally. Both can appear in casual speech with clothing sizes. Examples:
Który rozmiar wolisz – S czy M? (which of these)
Jaki rozmiar wolisz? (what size in general)

What case is rozmiar in here?

Accusative. The verb woleć takes a direct object in the accusative. Since rozmiar is masculine inanimate, its accusative form looks the same as nominative: rozmiar. You see a change with masculine animate nouns: e.g., “Which dog do you prefer?” → Którego psa wolisz?

Does który have to agree with the noun?