If your not familiar, a subwoofer is a component of a loudspeaker that produces a low bass frequency. You can choose a big or small subwoofer. Size doesn't matter as long as it gives me a theatrical-like surround, it'll make my day.
I recommend JBL JRX218S with 18 passive compact subwoofer with 1400W peak power handling. It's super sound in high quality! It's very affordable and easy to mount.