It all depends on how you want to listen to your music. If you want clearer sound, a smaller sub will work better. Smaller subs move less air, which generally results in a cleaner, crisper feel to the music. If you just want a minor upgrade to your music, anything from the 6 inch to the 8 inch range will work. If you want more bump but still want clean sound, 10 inch to 12 inch subs are great. They provide a solid boom, but don’t overtake the overall sound of the music.
This works really well for bass-heavy rock and hip hop. 15 inch subs are ideal for people who want to feel their music as much as they hear it. These subs can provide the bone-shaking boom that will announce your approach from a block away. If you want the loudest boom possible or are looking to participate in competitions, 18 inch subs are the way to go.