What size subwoofer to get?

Stehno is absolutely right. The cone size is not important. Since your current main speakers go to 35Hz you'll need a sub that can crossover below this level (unless you filter the signal going to the main speakers).
My main speakers go to around 35Hz and I'm crossing at 27Hz to avoid boomy mid-bass !

My suggestion would be to look for a used REL strata. It crosses very low (down to 25Hz), it's adequately powerful for all but the largest room, and if you don't like it or ever want to upgrade they hold their value and are easy to resell.

IMHO Better no sub than a cheap sub for 2ch music.

Socrates .. I disagree with some of your suggestions:
1) If speakers are rated to 35Hz, and you cross below 30Hz (it is usually recommended to leave the main speakers unfiltered, and cross below their cutoff frequency ala REL, ACI) then this will have no effect at all on the stereo imaging ... such low frequencies are not directional.
2) Of the several hundred CDs I have at least 75% benefit from the subwoofer, whether it be added weight in the bass drum kick, the sense of the scale of the venue on live recordings, timpani on orchestral music ... I could go on. The type of music that DON'T benefit are extremely limited. Of course the effect is most pronounced with electronica and organ music, but it is very present in most other types of music.
3) Separating the source of sub-bass from the main speakers gives flexibility in placement to allow good imaging on the mains, and good room loading on the sub. Full range speakers don't give you this option.
4)I have heard good full range mains ... in a large room they work very well. I have heard a good subwoofer ... in most rooms it can work very well. Both options are possibilities. However you seem to rule out the subwoofer approach without having had any first hand experience with one. When you say you decided .. was this based on listening, or just a preconception ?

I have Spica angelus speakers with a REL strata. The amp, crossover, and subwoofer is all one box. Total cost $500 for the Spicas and $850 for the REL. I don't think I could beat the sound (imaging and bass depth) for $1300.

I think our differing opinions come from comparing one-box subs, crossed below the mains to multi-box subs with external amps and crossovers. A one-box-does-it-all sub like REL or ACI contains the crossover, amp and sub, for anywhere between $800 and $1500, and gives bass of depth I have not heard on any main speakers under $2500 a pair. Creating a sub with separate crossovers, amps and cabinets I have no experience with, but it sounds like your opinions would be more applicable in this case.

How did I conclude you had no first-hand experience ... I read "and decided it was far more cost effective, and would also yield overall" and the wording suggests a decision based on presupposition, rather than a listening comparison.
Also you wrote "As I understand it you should crossover ..." this gave me the impression that you had never actually setup a sub. I guess you post was open to interpretation and I made the wrong interpretation.