Important a good screen, so no glare, and with sufficient resolution.
Brightness is measured in candelas per square meter (cd /m2) the more the better. Ideal - AMOLED or retina
On account of the size don't know, I would like to hear the opinion of used 7" and 10"
Only ten, seven is nothing like a big phone. You can use 9" being a iPad, and have kitaeveda with this retina.
In addition to screen, other specs are not important
You need minimum HD, and at 10' you need a large resolution and pixel density of PPI as they will be seen.
from the battery more than a few hours is not necessary,
8000 mAh at least given the fact that akum is rapidly degraded
Here's the problem, as they mainly China and of poor quality. market.yandex.ru