In terms of size, can't be big enough but this is mostly an economical thing if prices are twice as much for 2 inches more, I wouldn't buy it unless I'm swimming in money. If you feel the need to show off though and you can afford it, spend as much as you like and feel good about yourself
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The resolution doesn't need to be astronomical, it should fit the size, if you have sharp eyes you can go for higher resolution.
Contrast of course is also the more the better, but I generally have no idea what the figures really imply.
The way I do this is, I decide on a price I am willing to pay then I go to a site that quotes best prices and read all the reviews of the models in question.
I usually focus on the bad reviews, but don't overdo it, because there is always someone who is unhappy with a product because of their higher standards or they just want something to moan about.
Good luck.