I would not worry about your room and go for bigger speakers. I am sure I am in the minority but a lot of times I think a smaller rooms sound better (I will explain my opinion below).
But first we all need to stop making blanket statements on these forums as start thinking about the physics. For example a 40hz wave at 80db from a 8" driver is the same as a 40hz wave @ 80db from a 10" driver. The difference is the larger driver will be more efficient and will most likely have less distortion.
Smaller rooms always sound more immediate and intimate to me. The only thing you really lose is soundstage and imaging (depending on the dispersion of the speakers). I value dynamics over soundstage and that is why I often think smaller rooms sound better.
The problem with small rooms is not the size of the drivers in the speakers but how low they reach. Bass gets pretty room dependent down low. Plan on about 4db-6db of bass gain from 20-80hz. So if the speaker starts out flat at 20hz it is going to get pretty bass heavy in a small room. But if a speaker is down 5db at 20hz is may sound pretty flat at 20hz (80hz traps will almost always help).
If you want good examples of this look at Kefs 207/2 Stereophile's "acoustic" and in room measurements. The Kefs are shelved down almost 7db from 40hz-20hz. But the in room is razor flat (one of the best I have seen). Then look at the "acoustic" and in room measurement Revel Salons, a speaker that goes acoustically down to 20hz but in room can get pretty lumpy. Having the back of the room open will not really help you much. The closest boundary is what really matter, front, side, and ceiling.
On the subject of seating distance. The only advantage of sitting farther back is driver integration. The total spacing of the driver will be the biggest factor. Adjustable driver housings and coaxial designs are ways around the problem. But really closer is better (with in reason). You hear more details and the soundstage will be wider/taller, all in all the music will sound larger (think of how big headphones can sound).
For the speakers you have listed I have only heard the Thiel 3.7 and 802Diamond (actually heard them in the same room/day). I wound not recommend the 802Diamond with your room. This speaker is already bass heavy and your room will only add to that. I have also heard the 802D (old model) in about 5 different rooms. They really sound best in a large rooms because of there tonal balance, room gain will only make them worse. The dispersion is also not very even and with the close side walls the in room tonal balance may be off in the mids as well. On the other hand if you like massive bass, you may like the 802diamond in your room.
Now the 3.7 is a different animal altogether. The tonal balance is flat to lean and could use a few db of gain in the bass. These will sound very good in a mid sized room, if you accept the fact that they will never meet their soundstage and imaging potential. Now Thiel recommend 8-12 feet as a seating distance to get time alinement. If you are sitting at only 7', sitting lower or raising the front spikes will help. But really sitting at 7' the Thiels will still be much better integrated than most floor standing speakers on the market anyway...
PS, I have always found sealed speakers to work best in small rooms. Port loading can get tricky and it is one less thing to take into the equation. The 3.7 would be quasi sealed and I believe the Andras (at least the newer ones) are sealed.