First of all, welcome to the dark side.... Apple has been making great products for my uses, although at times with odd quirks, that one has to live with.
I have the 11" 2011 and 13" 2010 laying around. I have the (idiot) luxury of picking the one I like/need/suits my needs best.
I mainly use the 11" for these reasons:
- 13” has 30% more weight;
- 13” has 30% more footprint;
- without Bluetooth, wifi and with the brightness down to 2 or 3 notches I can get 6 hours work out of the 11”, sometimes even more. That’s enough for me;
- the performance of the 11” is stellar, compared to my use (as a very expensive typewriter 90% of the time);
your questions:
- 9 hours is possible with great care on the 13";
- 6 hours is possible with great care on the 11";
- for most people the times Apple state on their website are quite well possible, so that is 7 hours for the 13" and 5 hours for the 11";
- .docs are fully compatible if you use Office for mac;
- your looking at quite an expensive typewriter when using just word for 90% of the time
Just one question you need to ask, do you work 6 hours straight, or do you work for 6 hours and drink some coffee, look at the people around you, think hard an long about the next sentence you're about to write

For 6 hours straight the 11" is perhaps not the right one to pick, if you do other things during the 6 hours work the 11" might be right.
I hope for the 10.7.2 update to make my MBA even less power consuming. Leaving mail open at this moment brings batterylife down by 0,5-1 hour. That is a bit much. Overall I love the 11", I can take it in a small unobtrusive bag that doesn't yell "laptop inside" the light weight makes it much more fun to walk around with it and not being hindered.