What size subwoofer do I need for my room?

Size to cubic feet, not to enthusiasm. Under ~1,500 ft³ (a 12×14 bedroom): a sealed 10–12" is plenty, and room gain does the deep-extension work free. 1,500–3,000 ft³: a 12" sealed or ported. 3,000–5,000 ft³: a ported 12–15" starts earning its box. Over 5,000 ft³ or open-plan: consider dual subs — two moderate subs smooth room modes better than one monster.