17cm both front and rear (components as standard, but if you want an easy life, you can disconnect the tweeters and just install co-ax).
Just be careful about depth, can cause problems with the grilles going back on (which might not bother you!)
In terms of subs/amps there's two schools of thought regarding power, but the one I subscribe to is that you should ensure you have an amp that is at least as powerful as the RMS handling of the subwoofer - it's unlikely you'll kill a sub that way, whereas overdriving an amp will!
The most crucial thing is to not be swayed by claimed power outputs, a few 'rules':
1). RMS or nothing. If something quotes 'max' or 'peak' power then take it with a huge pinch of salt, these figures can be halved, then halved again to get the approximate RMS output, which is more of a real world figure.
2). 12v NOT 14v!. You car runs a 12v system right? Well be careful, many manufacturers inflate their power outputs by running the gear at 14v!
3). Resistance. If you're getting a 4 ohm sub and the amp produces its best power figures at 2 ohms, then it's not the right setup for you.
4). Cabling. Run RCA's and power supply wires down seperate sides of the car, this will help reduce interference. Keep all cables as short as possible and do not be tempted to loop up spare cable.
5). Grounds. Make sure it's a good earth and always scrape paint away to reveal bare metal - painted surfaces or primered surfaces aren't a good conductor!
6). Budget for cables. Don't forget that a good amp kit is going to cost a few quid. Don't go silly, anyone that tells you they can hear the difference between a £50 amp kit and a £120 on anything less than a competition system is imagining it. As long as you have a decent, chunky and well insulated set of power cables, and good quality screened RCA cables, you'll be dandy.
Just something to get you up and running!
Cheers,
James