What screen size do you imagine you'd need to work comfortably with it? For example, 13" is fairly small, portable and clocks in a few hours (uncertain about the whole 6 hour charge, that could be a bit of a stretch), but the small screen size may make it difficult to work on, since everything will be tiny and you might need to scale stuff up a bit to be able to read comfortably without straining your eyes.
To conserve a bit of battery, consider a computer with lower rather than higher resolution (15" screen with 1366x768 resolution for example doesn't look all that terrible), since a lower resolution essentially means it'll use less power, and the laptop in itself would probably be a little bit cheaper. Simply put, the larger the screen, the more battery it will consume, so you need to find that trade off between screen size and battery time. I'd suggest prioritizing screen size, as it's not worth ruining your eyes for that little extra battery time. Better to find yourself a wall socket, if possible.
