I have found no correlation between size and performance in subs. My subs have 10" and 12" drivers and go very low. The idea of a big slow cone for a woofer was prevalent when I got into audio in the early 60s. Then you had the Electro-Voice with a 30" woofer and the Hartley with a 24" woofer. 15s were common in 3 way systems. As speaker design has progressed woofers have , by and large, grown progressively smaller. Now the $33,000 Avalon Diamond , which may be the best speaker in the world, uses 2 7.5" woofers. It is the overall design of the sub that counts , not the diameter of the woofer. I would not worry about mixing different sizes, AS LONG AS, you are crossing them over sufficiently low. I use 22 Hz on mine. If you could hear a difference between them then you would be crossing too high, you should NEVER be able to hear a sub, just feel them.
Stereo Subs - Do they need to be identical?
I'm thinking of getting a couple of subwoofers to mate with my MBL 121 monitors. Generally, I've seen setups where people use two subs of the same brand and size. I'm wondering whether it can make sense to use two different sizes. For example - a Venoyne DD12 and a DD15. I'm likely going to go with Velodyne DD series or JL Audio.
In particular, I note that the big Velodyne 1812 uses an 18 inch and 12 inch woofer, with suppsed benefits in terms of speed (12 inch) and extension and depth (18 inch).
In particular, I note that the big Velodyne 1812 uses an 18 inch and 12 inch woofer, with suppsed benefits in terms of speed (12 inch) and extension and depth (18 inch).