What size subwoofer to get?

Slightly different advice:
1. A floorstander's low frequency (in this case 35Hz) can be expected to be its -3dB point (unless you're quite close to a wall). The crossover on a sub is its -3dB point. Therefore, a crossover point an octave above the low frequency spec point of your speaker is a good place to start. In your case that is 70Hz.
2. Though there are many design factors in subs, the amount of air moved by a large sub is much larger than a small one, requiring less excursion (often meaning less distortion) to move the same amount of air.
area = 3.1416 * r squared
10 -> 79 sq in
12 -> 113
15 -> 177 (two 10s)
18 -> 254 (two 12s)
3. Finally, a phase control as well as a volume control is useful in getting floorstanders and sub to sinq.
Good luck.

Stehno hit it on the head.

Rives also brings up some important points. One sub IS harder to place than two. On the other hand, two well designed commercially built subs can be quite expensive.

Just keep in mind that a vent of any type ( ported, passive radiator, slot loading and yes, even transmission line ) will have slower transient response ( reduced impact, less definition, increased ringing, poorer decay, etc... ) than a well designed sealed box would offer. The sealed box will also TYPICALLY have less problems with interfering in the upper bass since there is no leakage through the port or passive radiator "talking". Placement should also be easier since you only have to worry about room reinforcement / nodes from one radiating surface ( unless you have multiple drivers ).

The major drawback to sealed woofers is that a good one will be quite large. Having said that, two good quality sealed subs that are offer large box volumes would be VERY hard to beat if properly set up.

As to the Bag End, it is equalized to produce 8 Hz, but only at very low volume levels. As driver excursion and spl is increased, the bass processor rolls off the extreme lows so as not to muddy the more prominent notes slightly above this range. Kind of like a smart "loudness" control that one can find on receivers, etc... The biggest problem with Bag Ends is the construction of the cabinet, which is not as rigid or damped as it should be. It also takes a pretty good amount of power to really make this thing sing, which is true of almost all subwoofers. Sean
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How important is control of the rollof slope of the subwoofer? I had a REL (Q150e) that allowed me to control both the Xover frequency and the rolloff slope (6, 12 db, etc). That seems like a great advantage - is it really???

I'm using a Rel Strata III with Magnepans with excellent
results. The Rel is very tuneful, with a highly adaptable
crossover network. If will go down to 18Hz flat - how
much music is below that? I also agree that two smaller
subwoofers is usually better than one larger unit.

System-matching and your music-HT bias are critical. I use a Paradigm Servo-15 to supplement (not crossover) my B&W Nautilus 802's for the sub-50hz information on some (not all) music listening. The Servo-15 is an excellent HT sub and a good music sub. However, it's still too slow for the fast 802s on some music. Several knowledgeable "golden ear" audiophiles I respect have independently told me that there is nothing better than the Vandersteens for music (preferably using 2 or more). They each use 3 8" drivers and are very fast. I understand a cheaper alternative is NHT Sub Two's (2 10" woofers each), which are out of production. Though the Paradigm Servo-15 is an excellent compromise for music and HT, it just illustrates that you should be biased in favor of which is more important to you.