The modern stereo system

You will want more than 25 WPC for any serious volume.

For 10's or 12's I'd suggest 80-100 WPC
Sorry, but I am seriously LMAO at that. Totally anemic!
Triple the main speaker power and double and a half at a minimum for a single sub.
(She did say she likes the bass thump.... AKA LOUD!)

In my Magnum, it’s 90 watts RMS/180 peak @ the rear deck 6X9 Infinity Kappa’s. The sud is a 10 inch Pioneer at a RMS of 150 watts/300 max.

As your Pop probably said, “Son, if you’d goona hit it, hit good and hard!”
:rofl:
Why does a ported sub-woofer box have to be any larger than a sealed box? If it's ported, I would think the size of the box is irrelevant.
Actually a speaker will have specs that will dictate the size of the box needed. Sealed & ported boxes may vary a little or a lot depending on a few factors. It really is a tuning thing.

Radio Shack has a book on it that is actually really good.