What Size Subwoofer Is Right for Me?
The size of subwoofer is a very highly debated topic and ultimately comes down to what you are trying to achieve with your setup. Since subwoofers come in different cone sizes, heights, volumes, etc, there is no one size fits all.
In general, if you are going for lower-end frequencies, it is recommended to go with a larger subwoofer like a 15" or 18" subwoofer. Conversely, if you are looking for better replication of higher-end frequencies, typically an 8" or 10" subwoofer. However, the most popular size of subwoofer is the 12" subwoofer as it tends to offer a balance of both.
In some cases, your application or vehicle is going to determine what size of subwoofer is going to be right for you. For example, Skar Audio's VD Series subwoofer is one of its most popular lines as it is a shallow mount subwoofer and can be used in areas where there is not a lot of mounting depth like an under seat truck build.
Finally, some individuals look at the music they listen to and the dynamic range of that music. If you go for more of a digital bass style of music (i.e. Rap, Hip Hop, EDM) then you are going to want something with a little larger cone size and construction geared towards SPL. For those who like more of an instrumental bass style (i.e. Country, Rock, Metal), you may want to look into a smaller cone size for more accurate, responsive bass.