I would think that two 10"s would do the trick. You'll need to get a pair of 2 ohm DVC subwoofers, and to wire them in series to put a 4 ohm load on the amp. Then, the amp, will output 625W to each woofer.
http://www.kicker.com/06/tech-support/manuals/manuals/2008/2008%20ZX%201000%201500%202500-1%20e01%20web.pdf
I'm thinking now, after looking at the specs on the Kicker amps, that the zx2500.1 is just too much.
You see, you could get the zx1000.1, with two
4 ohm DVC models and wire them in
parallel. With this setup you'd end up putting 500watts to each woofer.
Find out what difference in price is between the 2500 and 1000. For the sake of 250W, it might be more cost effective to get the 1000. See if you can get a shop to hook up each setup separately, and have a listen. Maybe the 2500 would sound better. I don't know.