What size subwoofer to get?

This question is for 2 channel stereo use 50/50 rock/classical music. I have floor standing speakers rated down to 35hz on axis and instead of getting expensive larger speaker want to look at adding "powered" subwoofer to slightly extend bass. Three models are available:
10" woofer 30hz
12" woofer 25hz
15" woofer 22hz
All these models have adjustable crossover frequency cut-off
50-150 as well as adjustable volume. What size is best to get, my concern is getting too much bass that is hard to dail back and becomes overwhelming. Or is it better to get largest subwoofer and not worry, volume control will be able to effectively tailor sound and control bass?

This is my first subwoofer so need some general advice from members who use them, thanks.
Gunbei: I think you are headed in the right direction. Subs in the corner behind the speakers can and often do work quite well. It's not as flat a response, but if you keep them within 1/8 of a wavelength of the corner it minimizes the peak/node effect to some degree (eliminates the re-enforcement of back waves coupling or decoupling to the source wave. As to downfiring or forward firing--it makes no difference at crossover points below 65 Hz or so. Above that you might get a very slight difference. You may want to go to our website www.rivesaudio.com and go to the listening room (under acoustic issues). If you click on speakers there, there is a link that you can download a white paper on speaker placement. It goes through all channels and subwoofer placement. Hopefully, it's helpful.
Sean my experience coincides with your advice. I am using ProAc Response 1SC monitors which probably extend down to 40Hz at the most. But I find I must set my Paradigm X30 to it's lowest setting of 35Hz or the excessive overlap at the crossover point begins to muddy the midrange and upper bass of the ProAcs. That's where an extra control for shaping the slope of the lowpass filter would be helpful in giving more control over the rolloff.

Rives, thanks so much for giving me the free advice. I did check your website and found lot's of helpful information. I've been reading around to become more aware of some of the often used guidelines and from there I intend to do a lot of experimenting. I just wanted to make sure I started on the right foot.
Your speakers are very good and extend into the bass regions quite a bit. You do not want to lose any of the benefit of your speakers and there is no reason to try to redesign them by using a traditional, conventional subwoofer. Buy a REL (the size of model depends on size of listening room) and run your speakers full range. Then run the REL off your amp (it draws no current and won't affect anything) and then experiment with the lowest x-over setting you that works. I have ProAc 2.5s and have my REL x-overed at 28HZ. This is a major difference btw RELs and most other systems. I tried, at home, three other subs costing btw 1,200.00 and 2,400.00. The 1,300.00 REL Strata III was far and away the most musical and blended seamlessly with my speakers. You are right to want all the low frequencies you can get, makes a big difference but make sure it's done right. REL is the only make I know that does it right. I've heard Vandersteen's sub does same thing but is far more difficult to set up and adjust.
Since you are looking for music rather than Home theater bombast....try the REL....it comes up under your mains and works off the amp speaker connections.
To add to Rives' comments, front loading and downfiring will have DRASTIC effects on bass output and pitch, even if crossed over at or below 65 Hz. I would suggest listening to the two different designs side by side to see which you like best. My experience is that front firing will typically sound more musical with better pitch definition whereas downloading will provide greater "oomph" with less attack and tonality.

One should also take into account that downloading places more stress on the driver itself ( the suspension can sag pretty drastically over time ) and a system of this nature will require a slightly sturdier amp with greater current capacity / higher damping factor. Sean