Samsung's Tool:
http://www.samsung.com/us/television...VSelector.html
Recommends 51" for a wall 10' wide, 8' tall, and 8' viewing distance (assuming it's on a wall mount).
Wiki's charts from different retailers/manuf:
http://en.wikipedia.org/wiki/Optimum...ewing_distance
That said, they show 8' is too close for 70". Peter's recommendationn of 60-65 is probably a better target size.