Something like 500W upwards is preferred. I believe busog na yun GT5 with around 300W of power.

And you can always pair any subwoofer naman eh. There's no such thing as impossible in pairing 10" sa 12" although going 2 10" and 2 12" for uniformity's sake is also better for the eyes.

As for the dimensions, can't help you there. Try googling a ported box calculator since that's how shops compute theirs (and through their experience)